I always use salted and not brined casings, but from what I've seen they usually are good for 3-4 months, I've even found one that claims 6 months. If you need to keep them longer I recommend the salted ones.
For what it's worth, just in case anyone else is wondering about this, I found this on Allied Kenco's site. The hank I bought from them is a DeWied ez pre flushed brine soaked hank, and AK says it's good for "up to 6 months to 1 year". From what I've read they don't really go bad so much as start to develop holes.
I hope no one minds me posting such a long post (copy / pasting from AK) but this is probably as good of a place as any to paste it because Kevin has posted a great tutorial / reference, so here's a little more...
From AK's site:
http://www.alliedkenco.com/catalog/popup_text.php/fld/howto/tbl/howtos/key/17FAQuestions About Natural Casings:
How do you soak out casings and for how long
Hog Casings
Salted Casings: - For best results soak over night in cooler in water that starts out at 90º F (32.2ºC).
The fast soak: If in a hurry, follow these instructions but understand that you may not get maximum expansion capacity from the casing. Rushing the soaking process can result in the casing being sticky and they may not slide easily from the horn. This can result in breakage and sausage that is irregular in diameter or too small.
Rinse salt from casings
Soak in fresh water at 70º F (21.11ºC) for 1 hour
Soak in fresh water at 90º F (32.2ºC) for 1 hour
Place in fresh warm water 90º F (32.2ºC) at the stuffing table
Pre-flushed Vac-Pack: - Soak in fresh water at 90º F (32.2ºC) for 30 minutes
Place in fresh warm water at the stuffing table
Beef Rounds: - Soak overnight in cold water.
Soak in warm water 90-100ºF (32.2ºC - 37.77ºC) for 30 minutes
Place in fresh warm water at the stuffing table
Sheep Casings: - Soak in fresh water at 85-90º F (29.44ºC - 32.2ºC) for 30 minutes
Place in fresh warm water at the stuffing table.
 How long do you smoke sausage - The length of your smoke cycle becomes part of your sausage formulation. Your smoke cycle depends on the type of smokehouse you have and the type of product you are smoking. Contact your smokehouse supplier. Colored (Smoke Color )casings can help shorten your smoke cycle to increase smokehouse productivity.
How do you keep unused casings - Cover unused casings in brine solution or granulated salt and store in cooler at 40º F(4.44ºC) or less but do not freeze.
How tight should the sausage be stuffed - How tight you stuff sausage casings depends on the type of sausage and how it is to be linked.
For natural casings: When making a rope sausage, without linking, stuff to slightly less than the maximum expansion of the casings. If linking by machine, stuff 3-4 mm below the maximum expansion of the casing. Consult the instructions for the linker or your linker supplier because there can be significant differences in equipment. If linking by hand stuff 4-5 mm below the maximum expansion of the casing. Hand linking can put uneven stress on the casing. By under stuffing, you can reduce breakage during linking. Check the firmness of the link and adjust the stuffing pressure.
What is the shelf life of natural casings - Salt: 1 year or more
Preflushed in Vacuum Pack: 6 months to one year.
What is the best way to store natural casings - Store in the cooler at 40Fº (4.44ºC) or less in brine or well salted. NEVER freeze casings.
What is the most popular size casing for a fresh and smoked sausage? - There are no standards dictating the size casing to be used for a particular sausage. The size casing you use for sausage should depend on what you want your sausage to look like. The specification of the packaging and above all, your preference will influence the casing size . How many links do you want to make up a pound? How long must the links be for the packaging to be used? Traditionally, smaller sizes are used for fresh sausage and larger sizes for smoked sausage.
My casings smell bad; are they still good - Usually Yes. When your natural casings first arrive there may be some gas build up in the container, especially in hot weather. This can smell pretty strong. Kind of like a crowded room of bean farmers after a bean banquet. They are still good people though
What can I do to knock out the bad smell in my casings? - Usually all it needs is airing out. Leave the container open in the cooler for a while. Or, take casings out of the container and air them out. If it is really bad, rinse casings in fresh water, re-soak in brine and the smell will usually dissipate.
Putting baking soda in your soak water may also help.
What can I do to improve the bite on the casings - Cooking a sausage can toughen any casing. To maximize the tender bite of a casing, cook with moisture. Prick sausage before grilling.
Some casings are tougher because of their origin. They are usually cheap. The tough ones are usually thick and opaque. Smoke cycles can also affect the bite of a casing. Humidity during the smoke cycle is very important to maximize a tender eating experience. Consult your smoke house supplier about the best smoke cycle for the most tender bite.
What casing put ups do you offer
Salted, dry salted for longest preservation. Requires lengthy soaking before use.
Preflushed Vacuum Pack, casings are preserved in salt brine and packed in a vacuum pouch. Ready to use after rinsing off brine. Perfect for the average sausage manufacturer.
What is the length of a bundle or hank of casings - The traditional hank or bundle of hog or sheep casing was 100 yards. However, today there is no standard length. Some suppliers will base the length of the casing bundle on the price a customer demands. Now there are some casings like DeWied Processor Packs that will make about 100 pounds (45.3kg) of sausage no matter what size you use. That’s important for planning batch requirements and costing.
How many strands should a bundle have - The number of strands in a bundle depends on how uniform the diameter of the sausage must be and how long the individual strands of casing must be. The fewer strands and the longer they are, the less uniform the casings will be. In general, a hog casing will have 14-18 strands and a sheep casing will have 12-14 strands.
Why are the casings tough after cooking fresh sausage - Sausage was cooked in a pan too hot and too quickly.
Casings were not soaked long enough.
Origin of casing.
Sausage was under stuffed
How can I make my casings tenderer - Soak casings longer
Add lemon juice or pineapple juice to the soak water.
Use proper moisture levels during smoke cycle.
What is the webby looking spot on my casings - This is a patch of peyer (Pie-air) which is scarring resulting from the cleaning process and the removal of lymph nodes. All natural casings will have some scarring.
Why do my colored casings get dark streaks when I smoke them - Dry cycle was too short
Showering with water containing chlorine or high mineral levels.
What is the difference between hand pulled and knife cut casings - North American hand pulled casings do not have threads of connective tissue on the outside (Called whiskers). They are delicate and usually have shorter strands than knife cut. They may have more holes or weak spots. Knife cut casings have the small threads of connective tissue (Whiskers). They have an extra membrane for strength. Their strands are usually longer and have fewer holes. The threads of connective tissue on knife cut casings will melt off on smoked or cooked sausage.
