I'd like to get cheek implants but I'm worried about the long time effects of bone resorption. Is there a limit to resorption?

 Long term  experience with cheek implants.  if you see a very experienced cheek implant surgeon you should have no serious complication. Having done these for 45 years I can tell you that the only time with a surgeon would have a problem with these causing bone resorption is if they are put in under tension and are too large for the face. An experienced surgeon will avoid these issues. You should have a very good results but examine the surgeons before and after photos that are long term. 

The concept of 'erosion' with facial implants is an erroneous one. This implies an active inflammatory process as a reaction to the material that will not stop until the implant is removed. Rather almost all facial implants will undergo the normal biologic process of passive implant settling, whose amount is based on facial bone location and implant size. This is where the pressure caused by the introduction of a 'spacer' (implant) is relieved by self-limiting changes in the surrounding tissues including the bone. Thus the implant can be seen to have 'settled' into the bone a bit as a result. This is not anything new or extraordinary, it is a simple biologic process that occurs with implant placement everywhere in the body. I have seen almost 1000 facial implants removed/exchanged and/or replaced and I have never seen any inflammatory bone erosion or the need for bone grafting. 

Bone is living tissue and it remodels. We have been performing cheek implants for over 25 years  have never seen bone resorption be an issue