Before a hen even starts laying she builds up a reserve of fat. That fat is what she mostly lives on while she is broody so she doesn't need to spend a lot of time off the nest eating and drinking. Broody hens have been doing this for thousands of years without people helping them.
I've seen broody hens come off the nest twice a day for over a hour each time. I've seen broody hens come off the nest once a day for about 15 minutes. How warm it is can have an effect on time off the nest. But I've also had broody hens I never saw come off the nest. But since they are not pooping in the nest I know they are coming off.
You said you wanted to put some baby chicks under her and
@BlueBaby was nice enough to volunteer to help. See the benefits of putting your location in your profile! It's a shame more people don't do that. I've had broody hens that have only been broody a couple of days try to take over raising another hen's chicks. I once had a hen that had only been broody a couple of days refuse to accept chicks I put under her. I have had one hen that had been broody for three weeks refuse to accept all the chicks I put under her, though she accepted some. Usually that is not a problem at all but that one time it was.
The point I'm trying to make is that each hen is different. You can try earlier than 3 weeks and it might work. You can wait three weeks and it might not. You don't get a guarantee either way. Personally I'd be willing to try it earlier if chicks are available. But make sure they are really young, your odds of her accepting them is better if they are one to three days old rather than 4 days or more. It's not just that the hen has o accept them, they have to accept her. If you wait too late they may have imprinted on something else.
80 degrees isn't that hot, you will probably soon see some really hot weather before too long though. How well is your nest suited to hot weather? Will it become an oven when it does turn hot? Is it in direct sun or a shady spot like on a south or west wall? Do you have reasonable ventilation up high in the nest so hot air can rise and escape? Heat can be a real health issue for a broody, much more than her not eating or drinking as much as you think she needs to.