I'm really not trying to be a smart ass.... But it depends on where you start and where you are going. Look at this map:How long IS the run from dock to dive out of Coz? Time/distance range is all I'm looking for.
cozumel reef map - Google Search:
The island is 34 miles long.
If your diving Southern sites, and staying at Aldora Villa (North of town) I think 30-35 minutes or less. From the Aldora dock downtown, 20-25 minutes at the most. If your staying at one of the Southern Resorts, less than 10 minutes depending on the site and resort. Those would be for the farther South sites like Palancar.
Dave can provide a better answers than I can, I've never timed the boat rides. If the boat rides are a concern, just email them, or the operator of your choice, and ask for times to a specific site. It may also make a difference where you choose to stay......but that another issue.
Good luck, safe travels,
Jay