You can visit Borobudur and Prambanan in one day. It is a long excursion day that can take from sunrise to sunset. It takes about one and a half to two hours to travel between Borobudur and Prambanan. This doesn't seem like long at first, but rising early to watch the sunrise at Borobudur and then touring the largest Buddhist temple in the world takes its toll. While you can tour Borobudur and Prambanan in one day, it is easier to break it up into two separate days.