Every tea house has a main dining area/common room where you eat dinner and hang out by the fire. It's warm, cozy, and lively with trekkers . . . until 9 p.m. Most common areas close around that time and the owners usually lock the door after everyone leaves for their rooms, which means 9 p.m. is your absolute latest bedtime.
In our group, a lot of people went to bed around 7 or 8 p.m. after long days of trekking, but I usually tried to push it till 9 p.m. every night. There's nothing worse than going to bed too early and then being wide awake at 4 a.m. and having nothing to do but lay in the cold.