My personal goal is to get to 25% ROI, at which point I feel "safe". I feel from that point on, I can at least break-even on selling parts and what not, and all my rigs were just a fun hobby. Hard to say of course, since the market would be flooded with people selling off their gear, but we'll see just how much longer GPU mining can hold out for.
I think this will be completely dependent on your price of power. It can range from 5 cents to 40+ cents per kwh depending on where you are, and that can be enough to make GPUs profitable for many months to come, or not at all right now.