The travel distance between Zagreb and Island Rab, is about 22 km. majority of the route goes via the Croatian highway, where toll fees apply.
During summer season there are several direct buses between Zagreb and Rab, the trip including a trip with the ferry from Stinica to Misnjak. Travel for the buses from Zagreb to Rab is about 4-5 hours depending on travel route.
You can see current timetable here: