Hmmm, as a collector I've had one bad experience with commissioned work which took 3 years to appear but it did finally get to me
However, generally speaking I'm pretty relaxed about timeline expectations as long as the work eventually shows up.
As a dealer, 2 artists (Colin Wilson and Nicola Scott) whose work I handle accept payment after the pieces are completed (Colin does detailed pieces while Nicola does con sketches) while Ardian Syaf (too busy for commission work at present) took half payment up front for his commission requests. Never had an issue.