So undoubtedly, Obama will get the credit for having fixed it. Does that mean that (God forbid) Romney gets elected he'll get the credit?
The algorithm for figuring it out is quite simple, and is not time dependent.
Are things better than when Obama took office?
Yes -> He's doing a great job.
No -> It's Bush's fault
Are things as good as they should be?
Almost -> Obama's working on it.
No, they should have improved more by now -> It's Bush's fault
Assuming Obama is re-elected; when he leaves office in 4 1/2 years:
If things are better than they are now: Obama was a great president.
If things are worse than they are now: Obama was a still great president; it's Bush's fault.