Back to the original question about when to replace a synthetic sleeping bag.
The obvious answer is when it no longer keeps you warm.
But, if you want see how loft it has, and thus, get an indication of what temperature it's good for, take it to you local outfitter and lay it out along side a new synthetic sleeping bag. Generally speaking, if it's got as much loft as say, a new 20 degree bag, then it should good for about that temperature.