I would say that if your time is of any value to you don't keep the chicks beyond one week. You will essentially be earning $1-$3 per hour for all of the labor and feed/supplies cost you put into raising chicks until they feather out. Plus, you will lose money because people then will only be likely to want half of what you have available (the pullets) and you'll be stuck trying to give away free roosters, unless you don't mind butchering them for your own table.
If you are going to hatch chicks for someone then charge $1 per hatched chick, with a $5 non-refundable deposit, and get a signed basic agreement saying that any unclaimed chicks will be sold by the seventh day after hatching notification. Get it in writing and keep track of the birds.
I have done the hatching service myself and have never -- NEVER -- made a profit or anything close to it. But I have spent countless hours caring for chicks (feeding, watering, cleaning bedding, hauling wheelbarrows full of dirty litter to the compost pile, heat lamps -- in spite of broody hens, electric bills, trips to the feed store), trying to contact people who don't seem too interested in getting back to me in a reasonable time frame, waiting for people to show up to pick up their chicks (who invariably end up being an hour late), and then taking back roosters when urban farmers discover they can't keep them. The amount of time I spent was more costly to me than any money that ever touched my hands, and I'll never get that time back.
Sound bitter? Don't mean to -- just trying to portray it as realistically as possible.