When to let them on grass?

I have a 12 acre pasture for my 2 horses. There is a little bit of grass starting to grow, I was just wondering what would be the best time/idea on how to let out my horses?
I was thinking just let them out while I am there and let them graze a little bit and then bring them back down.

I think as long as you build them up to it gradually, you should be able to start out at 1/2 hour for a week, then an hour and so on. By the time you get them up to a half day or longer, the high spring growth sugar content won't be so potent and there shouldn't be any problems. This is all assuming that your horses are not overweight and already predisposed to founder or other sugar/starch related problems.