You should visit the museum first, then the Qianling Mausoleum, so you can stretch your schedule. What if you do it the other way around? The museum seems endless, with the tombs of Prince Zhanghuai, Princess Yongtai, and Prince Yide, and you'll need at least two hours each. What if you leave the Qianling Mausoleum for last? That way, you can still enjoy the scenery from the mountain. Just imagine.