On Domestic fleets, trips range from 1 to 5 days in length. (5 day trips only on former "north" metal - i.e. 320, DC9.) I would say 3 and 4 day trips are most common on former "south" metal. (73, M88) Day trips and two days tend to go senior in bases with a large number a pilots living there (ATL, MSP, etc.) "Short leg" aircraft (DC9/MD88,) tend to have between 2 and 5 legs per day, with the longer legged aircraft (320, 73, 75,) tending towards the lower end of that range.