I think that it takes approx. 3 h.
We would recommend you to go there for a whole day and stay at a hotel. You can do this as daytrip from Xi'an, but then, you'll only be on the mountain at noon, an then, everything will be soooooo crowded! (We made that experience when walking back to the cablecar-station in the afternoon - it was horrible!
By staying at a hotel in Huashan, we could start early in the morning and had the mountains to (almost) ourselves until noon - that was wonderful :)
Another highlight at Huashan is the "Plankwalk in the sky" - we will talk about this in our next post. This really is a must-do (even if nerve-racking)!