Shuttle runs 24 hours a day, departs from Arrivals level, well marked areas for Hotel vans on the second curb. The oversize vans are white and will say Marriott LAX on their display. Most of the hotels use the same contract provider, so the vans are fairly nondescript except for the LED-type displays. You won't have trouble spotting the right one.

Generally, anywhere from 1 to 25 minute wait once you are curbside.