They assign your first day of reserve at 9am 2 days prior. They assign each following day by 6pm the day before. I have seen trips added 4 days out and also assigned the same day. They try and be proactive. Just remember just because you are assigned flying doesn't mean you get to keep it. My trips have been pulled and my schedule has changed 4 times in a day. Reserve sucks and there are no guarantees other than what days you are on duty. That being said you won't be on reserve more than a month or 2. There were over 70 people below me by the time I finished IOE.