Will the wild grass/weeds most likely keep them busy?
Unless you keep them penned up, they will eat some grass and some weeds, and they will do some damage to the grass. But they do like to scratch in some kinds of places better than others. If they have a place they really like to scratch, they will destroy the grass more slowly, which gives it more opportunity to grow back. That means you have more time before the lawn becomes a bare desert.
For scratching and foraging for food, chickens seem to really like a spot that is shady in hot weather, a bit sheltered (trees or something overhead in case of hawks), has soil or mulch that is loose and maybe a little bit damp, and has plenty of tasty things in it (like worms or bugs.) A compost pile can be good, or the soil or mulch under trees is sometimes good, or of course they might be happy to destroy a vegetable garden or flowerbed.
They also like a place to take dust baths. That could be the same place I just described, or it could be a place with more sun, where the soil is more dry and dusty. I don't know what makes them prefer one over the other-- maybe the weather or personal preference or something else.
If they don't have places they like for scratching and dustbathing, they usually make some: probably in a place you don't want them
