Bad timing

I got an email the other day inviting me to review an article for one of the journals in my field. I was quite surprised. I figured that maybe my supervisor recommended me thinking I don't have enough to do with my defense being in a month. I sent him an email asking if he recommended me and he said he didn't. That was even more shocking. So someone has been keeping their eye on me or something to know my area of expertise and request that I review their paper. The other odd thing is that the conclusions they make in their paper are very similar to one of the unpublished chapters in my thesis, yet on a different gene. It's a bit eerie that they would request me as a reviewer on a topic so close to a chapter in my thesis when the only person who had read my thesis at that point was my supervisor. I also think it's odd that I would be requested as a reviewer since I'm still a grad student, abeit almost a PhD. I would really like to review the paper because I've never been requested to review a paper but I'm just worried I don't have the time. How long does it take to review a paper anyways? The email says they need the completed review back in two weeks so it wouldn't take longer than two weeks but how much time of those two weeks would it take?