My understanding is that the Bills will retain his rights indefinitely. He either plays for the Bills, or he just sits, unless the Bills release him which they have no incentive to do, or trade him (but we're way past the trade deadline for this season).
I'm convinced his retirement is due to his wife being unhappy with not moving back to Philly, where she was happy. Asking her to move to a new city she doesn't know, at the start of another typical Buffalo winter, probably didn't sit well with her.