It really depends, for me. Of course, for show season, I do a whoooooole big, long grooming session! Bathing, mane & tail conditioner, polish, etc. However, for lessons, I sometimes do a quick groom, or even a 20min one.
Usually, a 15-20min one. I pick the hooves, comb the mane, tail, & forelock, brush the face/body, etc. :) Plus, if Daytona rolls, the more I'll have to clean! :P