I believe 4.5 hours, easily. Just to remove the drive belt you need to remove front engine access cover, behind the seats. You need to remove the engine cover. Draining the cooling system, and finally removing the water pump. Then you need to put every back together and refill the cooling system, which looks like a PITA to get it right. I bet a do-it-yourselfer would take at least a full day.
It seems like everything but an oil change takes longer on a Cayman.