Kola's reply is pretty accurate. It would be about an hour ride from the Cancun hotel zone to the ferry dock at Playa del Carmen. The ferry crossing itself takes a little over 30 minutes, but the initial boarding, then docking & unloading everyone adds another 15 or 20 minutes to it. It felt like about an hour. There were LOTS of tourists on the morning ferry just going over for the day. Cozumel is so small, you could pretty much see all you needed to see in 1 day, and the ferries leave every hour, on the hour.
We took the ferry over to Cozumel and stayed 3 days after our trip to the Grand Mayan. I loved the snorkeling on Cozumel. It is much more like the Caribbean than typical snorkeling in Mexico. So, if snorkeling or diving are in your plans, I think a day in Cozumel is totally worth it. If you were just going over to see the city of San Miguel -- I would say "skip it". We felt that Playa del Carmen was 10 times better than any of the restaurants or night life on Cozumel. If you were in search of the best beaches -- I would opt for a day in Tulum instead. The beaches there are absolutely gorgeous -- right out of the Corona commercials. We felt the beaches in Cozumel were not as beautiful, but that the amazing snorkeling MORE than made up for it!
My hubby and I loved our 3 days in quiet Cozumel & the amazing snorkeling. However, our 13-year old has no desire to EVER go back, because she felt there was just not enough going on. She preferred Playa del Carmen or even the pool scene at the Grand Mayan -- where there was more excitement.
Just my 2 cents worth.
---Rene McDaniel