All you need to board is a full compliment of FA's. I have boarded a few times where they said the pilot(s) was on his way, but I never waited more than 10 or 15 minutes after departure.
We had a bit of an unusual situation this week BMI-DFW. Plane arrived about 20 minutes late from DFW at 3:00, so crew needed to do a quick turn to not delay our 3:15 departure too much. By 3:15 they were ready to board, but we had no crew at all. Turns out the hotel van had broken down. Crew arrived about 3:30 and we departed about 3:50. The crew delay plus a very long routing to avoid storms made us about 1 hour late, causing several misconnects (luckily not for me)