In general, you can keep longeing–changing direction at roughly the halfway point–if your horse’s trot and canter look increasingly relaxed and balanced and he is respecting and listening to the side reins, not trying to run through them or lie on them.
When he begins to breathe a little heavily, bring him back, let him walk a couple of times around, maybe canter a little more and you should be done.
From the December 2000 issue of Practical Horseman magazine.
