From Cancun City, buses depart approximately every two hours. You can find schedules & fares on the ADO website...http://www.ado.com.mx/wadod/compra.jsp
It's in Spanish, but another site called ticketbus has an English language option...
Cancun to Chetumal is about 300 pesos on an ADO express bus. The water taxi from Chetumal to San Pedro is about US$30. The ADO overnight express bus from Cancun City to Belize City is 496 pesos.