It is very hard to guess when they actually start the incubating process as the hen will spend long periods in the box without actually keeping the eggs at brooding temperature. You will probably notice a change in their behaviour in about 4 weeks when they will scurry up to the netting looking what goodies you have brought them.
A tablespoon of hulled oats soaked for 24 hours in a small container and rinsed properly before you feed it is appreciated. Some chickweed from a safe source, dandelion heads, groundsel. Any part of vegetables that you can eat will do them nicely. Use common sense and keep the proportions to what they eat and remove the uneaten food so it won't go off.
If you have two hens they may chuck the eggs out of the box after five or six weeks - don't ask me how they manage that as I've never seen them do it - and lay again. Providing they are reasonably healthy, laying eggs will not do them any harm.