Typically, the onset is fairly sudden measured in hours to days. This could be considered a form of depression. But we were all heavily screened psychologically as well as physically, so this was not a major problem. But I would add, everyone felt the isolation and darkness. Most people felt our productivity was much less than half than what we started the year by the end of winter.
Most people only visit for the summer or part of the summer which is four to five months long.
That's a rough estimate. Diversions were any indoor activity you could imagine. Including chess, darts, poker, cribbage. We played darts against another base using short-wave radio. Our video library had over 5000 movies.