I currently play in youth orchestra. Last season, I had a part in 3 of 5 pieces. This time, I only play in 2 of 4 (got a new piece just yesterday, so it used to be 1 of 3). The other two pieces which I don't play are a full symphony and another longer piece by Brahms. Usually, I don't spend very much time at rehearsal, seeing as the other pieces take up a lot of time. There's a rehearsal schedule posted and I'll get there about 5-10 minutes before I'm needed (after being tuned of course) and usually try and leave after I'm one rehearsing the pieces. Sometimes though I'll try and get there during the break, so that I can move my harp on stage without being disruptive to other members, and then just sit there with my book until I'm needed.
When you go to rehearsals do you usually stay the entire time? Or only about 40 minutes? At first, I was very frustrated at only having a part in one piece, and only staying about 20 minutes. Sometimes I wish I was needed more, but its just the life of an orchestral harpist.