There is not necessarily a good answer to this. I have seen stents have clot form in them as early as a month or so and as late as years later. When one is years out, generally the same factors that cause an artery to become blocked are involved in the re-occlusion of a stent. If one takes appropriate medications (aspirin, statin), does not smoke, controls diabetes, exercises, and eats properly, then one can reduce the risk of causing a clot in the stent.

If your question is more along the line, "will a cardiac stent break down?" then the answer is more generally no. Your blood vessels will grow cells into the stent and it will eventually become part of the wall of the artery.