I think you have to decide what you're doing. Have a cell phone on you, of course. If I decide I'm going to do ground work and conditioning in a familiar setting (just long trotting in our arena), then I'll take my chances and have a short session. But if I'm going to be pushing my horse or teaching it something new and confusing, or combating a problem like bucking or bolting, I wouldn't. If the horse is apt to be hot, I wouldn't, but if the horse is a dead-head, I would. Knowing your horse and recognizing the signs of a problem is the best you can do.