There is a lot to consider. I just checked my log book, and I found the following dives for comparison, both on June 29, 2005.
#1: Maximum depth 89 feet; average depth 47 feet; bottom time 52 minutes. Got on the boat with ~700 pounds in an Aluminum 80.
#2: maximum depth 48 feet; average depth 33 feet; bottom time 80 minutes. Got on the boat with ~600 pounds in an aluminum 80.
So what does that tell you? Less than you think.
The first dive was to a wreck (the Jane C in Aruba), and there is only so much time you can spend there. We came up to a very shallow part of the reef and explored it for quite a while. The first half of the dive was a somewhat active dive, meaning that in exporing the wreck, I had to use my fins and propel myself, sometimes against a mild current. The second half had a lot of exploring near the boat mooring until I had had enough--very gentle.
The second dive was a drift dive in a very mild current. I slowly worked my way to a shallower and shallower level by moving closer to shore as I drifted. At the end, I was exploring at safety stop depth and barely breathing. I only surfaced at 80 minutes because my buddy was low on air and wanted to go up.
Everything affects your time on a tank of air: your maximum depth, your average depth, how hard you are working, the water temperature. I have done dives on (for example) the Spiegel Grove in a raging current where my bottom times are not all that impressive.
BTW, I agree with all the breathe in and out s-l-o-w-l-y posts. Under the right circumstances, I almost try to feel as if I am falling asleep.