Mine took longer for the GB ROMs than FroYo, like 5 minutes or so.
If I were you I would delete all the files you are using and re-download a fresh copy of the stock GB. Then I would forget what steps you are using to install, and re-read all the instructions to make sure you didn't skip something.
After reading that it sounds condescending ... I don't mean it to be. My point is to start fresh. I don't know how many times my assumptions that I was doing it right caused needless repetition of the wrong process.
Sent from my Captivate running FireFly