Rule of thumb is to start with 15 minuets per day and work your way up from there. Especially with fresh spring grass that's hyped up with all sorts of sugars. Eventually you get to the point were you can leave them out 24/7 if you wanted to.
My own horses get several hours per day on grass, and my rescue on nice nights gets to stay in the pasture. Still, I would be wairy of letting them out 24/7, and wouldn't allow them more than a half hour increase per day to decrease the risk of colic and founder.
How lush is the grass? Depending on how large the area is, if the grass is lush and the horses are not usually on pasture I would not leave them out overnight.
Can I ask how much hay are your horses getting? I know in your other post regarding senior feed, you were trying to put weight on your horses. Although allowing your horses to graze is great, having substantial hay available will help them maintain a proper weight.
When I put mine on grass I leave them on 24/7 its just not practical to limit the time for a large number of horses I'd spend the entire day catching them. I make sure their full of good dry hay and have access to it then open the gates. We have 180 acres of pasture and let it get 6 to 8 inches high before we start grazing season. 100's of horses over the past 40 years 0 probelms so far.
We have 4 horses on a 7 acre pasture. I do the 1 hour increments until they are out all day. Since you have them on a small acreage it would be best to let the pasture rest for a few days a week. I have a 2 acre dry lot with lean-to shelters (and hay) that they go to in the winter and when its muddy or storming. I try to keep them off the grass if its soupy. Its better for their hooves and won't tear the pasture up so bad.
If you are up to 8 hours on grass I'd say you are ready for all day.
I agree, chuck them out there lol! If they are already out there for 10 hours a day then they are used to the nutrients and its grass they are already used to since its the same pasture. I've NEVER had any problems with it.