I would also emphasise importance of the connection with the photographer. That's very important because they capture the feel for what kind of couple you are and that affects the kinds of pictures you get back from the day- i.e. funny, romantic, classic etc..
I do agree with what Agape said, however, just as an extra point to consider-we were so glad we ended up going for 12 hours with our photographers, it meant they were able to capture every part of our day and we really appreciated that when we got the photos back and were able to see parts of the day that we had missed.
I paid for 5 hours, which covered before ceremony photos at church venue, ceremony, between, and 2-2.5 hours of reception. We made sure to do the formalities before he left. If I wanted prep photos at home, I'd have paid for 7 but I didn't. 12 would have been unnecessarily long for my wedding.