if its labelled chick started grit I'd assume it for use straight away. Alot of people just use sand. As for treats, I think about a week... some people start earlier though. Make sure there is no long strands of grass, and If your giving treats, their going to need some grit to grind up the foods with
That's more of an opinion question. Some feed anything and everything from day 1 cause with a hen free ranging they'd get all sorts of food. Others feed nothing until they are old enough to go with the flock. Others are somewhere in between with either a certain age they start treats or certain treats they feed to chicks. Everyone will have a different answer. Just make sure they are eating their chick starter. When you give something new they will sometimes fill up on only that which is especially bad if it's inedible like grit or sand.
If it's something that needs ground up, think about whether you'd have to chew it, they will need grit of some type. Chick grit can work for standard chicks but I don't know about if you have bantams. I can't get it here. The smallest grit I can get is barely small enough for my adult bantams. I just scoop sand out of my horse arena. I don't hatch large batches at a time so 1 cup often lasts until they move outside where they can find their own grit and/or use the grit in the coop.
I agree with the others here. A little common sense will go along way. I too found "chick grit" at the feed store and I offer it free choice and a little sprinkled over their food. Today I introduced worms and small crickets to my 3 day old chicks. 5 out of 6 loved the worms and crickets. There were a few crickets that were a bit too large to eat so I put them back outside. Like what Akane said, I like to try and think of what a free ranging hen would come across and feed to her young.