Yes even mainline is random. One week your trip could start on Thursday at 5 am and end on Sunday at 8 pm. Your second trip might start on Wednesday at 1 pm and end on Saturday at 10 pm. Then you may have 5 days off with another trip that starts at 8 pm on Friday and ends on Monday at 10 am.

Basically your start and end times are almost never the same. Some trips may run with all early shows on all 4 days and your next trip might have 1 early show, 1 mid morning (say 10ish) and the other 2 days might be late shows around 2pm +. Bottom line is it's random but if you’re a line holder then you were able to bid on your schedule so you will know what your month will look like well in advance. You’ll know when your trips start and end and if they are a.m. or p.m. trips.

Now if you're on reserve you're on-call and at the mercy of the company, after-all a reserve pilot is one of the most junior pilots on the list and expected to be used by the airline to fill in the gaps. You might have a reserve schedule that's Sunday through Thursday from 4 am to 4 pm or 9 am to 9 pm. You'll sit at home or at a "crashpad" until you're called in. Your airline will expect you to be fit to fly and ready to fly whenever they need you. After you are contacted you will normally have 2 hours to report to the airport where you could be scheduled to deadhead (ride in the back with the passengers) to a location and then fly one leg back. Sometimes you'll get 2, 3 or 4 day trips but usually it something out and back. Reserve pilot normally only get 10 days off per month where lineholders usually get 14+. Life on reserve sucks but it doesn't last, you will get a line sooner or later.

Also, as a junior pilot don't expect weekends or holidays off until you're at least half way up the line holder list. Until then you will get a random weekend off here and there but nothing consistent.