I don't know how you have arranged your transport from CUN to your hotel but Usually its a round trip and they will tell you exactly when they will pick you up. If you have not booked round trip transfer, i recc. you do as it is alot easier then trying to get around mexico by yourself. If you did book transport, I would just go ask the guys at the transport desk.

Just for referance:
The bus ride could be 1-1.5 hrs although that early their will not be traffic so it depends on how many people it has to pick up. Plus, assume you will have to be at the airport 2 hrs early because all the transport companies will not want you to be late so will always leave super early.

Figure a total of 3.5 - 4 hrs prior to departure, they will want you on the curb in front. Enjoy the beach, and you chose wisely not to stay in Cancun proper unless yourre 21 and want a loud party...