Horses can be out 24/7 all mine are. At this time of year the grass is starting to drop back and a lot of the goodness is gone, whatever it looks like. So, it is just your preference. I like Natural, with as little faffing around as possible, if you like playing , bringing them in and shoving them back out- it will probably not do them any harm, except maybe puzzle them a bit
Remember though, an hour on really truly rich grass and then brought into stand in a dry lot or stall, will actually do far more harm than 24/7 on poor grass they have to work at- it's the shock to the system that will start Founder, not just the grazing. Also, of course, a horse grazing is a relaxed, contented hors. So- what are your pasture amounts and conditions???
My little lawn mowers get out for a good part of the day. About 7 am to 5 pm or so. I move thier pen every few days to keep the grass down. With these gas prices why use it in the lawn mower when I have 5 minis that are more than happy to help out.