So many canals, so little time!
I feel like I ran around Venice in a big hurry for the 5 days or so that we were there. It's just that the city is so beautiful and there is something interesting to shoot everywhere. It's almost not fair that one place has so much amazingness. I need to get back there! Really though, you need only a few days to "see" it but at least a week to really photograph it properly. After 3 or so days of continuous movement and shooting you will find the spots you consider the best. Then, you will want to return a couple of times (or more) to each of them to get the best light. And just like that, a week is over. Maybe you need 2 weeks! ;-)
If you need a starting point, here is my list of the best places I found to shoot in Venice. Hope it helps and let me know if you have any questions!