We do the same all year, only with slight time variations - they're both turned out in the day and stabled at night. Although we have now let them into a bigger field with more grass; so they have both decided they want to come in at 5 now instead of 3 - which is good, as I get an extra two hours to get mucked out etc
The field at the yard where I keep him is an absolute bog so they're probably out no more than 5-6 hrs per day (less if I've ridden him) as there's quite a lot out there and after a while they start to mess around and fight, cue bites and torn rugs! Grass is trying to come through though and they will move to a different field late April.
Ones out 24/7, the other is a skinny tb who comes in at night durin winter. I cant wait till the weather turns now though, so I can bung them both out 24/7 on my summer grazing... Had enough of winter now.
You guys are lucky having your horses out until 5pm! On a college day I turn mine out from 5:30am and they're brought in at 1pm every day but in half term they get turned out at 7:30 til 1pm everyday. But then again fields are being rested and they don't want them churned up which is good in a way.