The way I apply smoke using the Bradley is judging the mass to surface ratio. If there is a lot of surface as oppose to mass, such as what you have with thin cuts of meat (ribs, chicken parts, pork chops etc), I use 2 hours or less. For cuts that have greater mass to surface ratio, brisket, butts, roasts I use 3 - 4 hours. But I do have exceptions, do to my particular taste. For pork loins, whole chicken & turkeys, and tender roast cuts I will keep my smoke time to 2 - 3 hours.
I take a slightly different view on the 140°F temperature, and that would be the surface temperature, not internal temperature. The 140°F is when the meat starts to denature, the cells begin to shrink and the texture of the meat gets firmer. It would make sense that if the meat properties change at that temperature, the reaction the smoke has with the meat will change.
It is true that smoke penetration is only 1/4" to 3/8" deep at ideal smoking/cooking conditions As mentioned above smoke will continue to adhere to the surface of the meat. Smoke will adhere to almost any surface, but the hotter the surface becomes the less amount of smoke will adhere. So the smoke flavor will continue to get stronger, as you apply it.