Like Mark and Rick said..... it depends.
We've made entire days at sites, Arrive right after low dide, engoy a skindive for 60-90 minutes in tidal pools, take a break have a picnic lunch, do a full tank dive on the approaching high tide, usually 40-60 minutes of bottom time maybe 80-90 minutes total in the water, get out have a snack, go back in just past high tide repeat on a different heading or plan. What I described was a pleasant 8 hour stay.
On the dives as mentioned we usually go down at about 10 feet, soon slope to 15-20 and then have the middle third of the dive in 25-30 feet, sometimes 40 feet. Again it depends on the site, tide and heading.
Sometimes we hit a site, pack up, go enjoy a nice meal, go to another site. it may be a mix of fresh or salt depending on location, tides and conditions.
I like doing 2 dive days, it spreads out the overhead of loading, cleaning and all. Before we invested in our second pair of tanks we would did hit a dive shop for refills between sites.
Sometimes we even sneak in a mini dive. Here in Maine it's serious wesuit country like many places. After doing a dive those lucky enough to have extra air will get out of the wetsuits, drop some weight and go back in in just swimwear for a frolic in 10-15 feet so we stay above the thermocline. Thats's a nice treat in the peak of summer! In the shallows I can often bag another 20 minutes though I haven't been logging those as dives I suppose I could.
Pete