score:3
I think you should be fine. The nearest manned station to Tanggula is Anduo (Amdo, ๅฎๅค), and that's only a 5 hour train ride. It's at most another hour from Amdo to Tanggula, so I'd say 6 hours is reasonable.
Source: http://search.huochepiao.com/shike_lasa_anduo
Also, I want to just say the scenery of that part of the train ride is actually fairly unremarkable. The train doesn't stop there, and the land's pretty barren, so don't set your expectations too high! The train ride itself actually is really interesting if you haven't been on a developing-country sleeper, though, so that'll be quite the highlight.