Ditto what Suki said.
I've been working on exclusive revisions with an agent since June 2009 (which included what I would call two and a half significant passes). We've agreed that this last one is "it", and he's on board with officially representing me. So once he signs off on this final draft he will send me contracts to sign and we will go on sub etc. It all took much much longer than a thought it would, but the book is also much much stronger than I thought it would be, so that's a nice trade off.
I suspect as you work with this agent it will become clear to you how involved they have been and how enthusiastic they really are for the work and hopefully you'll develop a relationship of sorts. As Suki said though, if things are not working out, or they are taking too long, etc., I think a polite, professional email asking them how much longer they expect exclusivity (or some such) is fair. But if you are interested in this agent try to be patient and remember that they have put in a lot of time with your ms as well (notes, re-reads, etc.).
As for time frames - in case it's helpful to anyone else going through this - and keeping in mind that all agents are different: My first set of notes (which were quite involved) took a brutal amount of time - almost two months to receive. It took me nearly three months to do that first revision and the agent read the new draft within three/four weeks but took another three to get me a second round of still quite detailed notes. That second pass took me a little under two months to complete, and he read the new draft within about a month. There was only one remaining sticking point in that draft and so the "final" draft only took about two weeks to turn around. I'm waiting for the (hopefully) final sign off now and it's been about two weeks...I expect to hear next week.
Good luck with your revisions - and I hope it results in a firm offer of representation! Let us know what happens.