So this sounds like, if a person had a trainer to work with they could maintain a TRAINED horse on their own w/ hauling to lessons on say... a weekly basis?
What makes that easy is having the trainers available, close by and the money to do so. (I found reining lessens around here average $50, still cheaper than keeping the horse in training full time.)
But the time spent with a trainer will depend on a couple of factors, how much maintenance the horse requires, even though one may be fully trained some are easier to maintain than others. And the time, effort, consistency and knowledge of the rider.
ETA- I forgot to add, you don't have to do one or the other(haul-in for lessons or kept in full time training). Know a few people that would put a horse in full training for a month or two then take them home and haul in for lessons. Did both so it was a little cheaper but the horse would go in for a "tune-up" as needed.