I agree with jwz, from what I have seen most sims will sign immediately if not being considered by a human coach. The only time I have seen them not sign is when the recruit was considering a human coached team and he filled his schollies. I believe the sim will sign the recruit next cycle if a human coach does not put forth enough effort to be considered by the recruit. They might wait a cycle and then sign, I can't remember for sure.
If you are going to wait for the last minute to try and sign one, you had better bring the bank all up front, especially if he has D1A sims on him. I have found it usually takes a few rounds to get them to consider you because they will turn down visits and such at the beginning. If you are trying to pick one up for cheap, it isn't going to happen!!