I try to unload my horses and walk them around every three hours or so.
Depends on when I can find a rest area or truck stop with some grass to
walk them. They need to stretch their legs and work the kinks out a
little.
Eight hours is way too long if you expect to do a ride the next day...
Also, I try to give my horse a chance to relax in his corral or on a
picket line the night following the ride unless it is a very short trip
home to his pasture (say, 1-1/2 hours or so)