If your hen is able to get up and out for potty breaks without your assistance (no door or wire that needs moved to allow her out of the nest) then I would allow her to decide after day 18 or 19.... if she is really easy going then you could encourage her to hop out for a particular tasty treat, but it isn't worth getting her freaked by trying to force her. Just make sure she has water and food within easy reach and allow her to choose. Be prepared with fresh bedding available in case she does broody poo at some point in the nest. It doesn't happen all of the time, but it does once in a while.
As far as eating... ours usually eat pretty well through most of their brood but often reduce amounts the last few days. I will put a small dish with bits of her favorites mixed with chick starter in the nest with the broody, tucked into a corner, sometimes they peck at it, sometimes they don't. If your hen's body condition is good then don't worry unless you notice a change. Fine chopped peanuts and hulled sunflower seeds and finch feed are a few that are easily given and can stay for extended periods without it going bad.
WalMart sells some small metal dishes in the pet department (for like a buck a piece +/-) I think they are intended for cats or kittens but they are a nice size for broody feeding and fitting into the corners of a nest box. I have a few of them in each size, the wider, flatter one I use for the new hatched chicks when they are still in the nest with mama (first 48 hours or so). The little ones can easily run in and out of them as mama begins to teach them how to eat.