I'm going for 2 day's in Feb with another 3 friends and we are always being told 2 day's isn't long enough to enjoy that wonderful city, let's see what others say !
You can do it justice in one day but we're all different and look for different experiences on holidays. I guess if you are in a hotel in the city you could spend less time trawlling about Venice and maybe relax at the hotel pool before dinner, we travelled in both days (from Austria then from Slovenia) and felt one day wandering about was enough.