I bought 15 week old chickens almost 2 weeks ago and I just checked my not 17lb bag of food but 25 lbs of food.
Within the 2 weeks I'd have to guess they ate a good 5 lbs of it. Of course I'm just guessing I picked it up.
The bigger they get the more often I have to refill their feeders.
If you're getting young ones I'd get the 50lb bag.
Buy the 50 lb bag, it will not go bad(before 25 chicks have finished it) then the next 50 lb bag will go a lot quicker, some sell 5 and 25 lb bags, but you PAY for the convience of getting a small bag, I know 25 lbs was $8 and 50 was $11.
We use Blue Seal OrganicLife Chick Starter. It's certified organic and non-medicated. it only comes in 50# bags but I foudn that even with just 5 chicks we went through that by about 7 weeks and then switched to grower.
Locally I can also get Nature's Best Organic Chick Starter and a Poulin Feeds organic by Green Mountain Grains that contains flax seed. I wish I had known about that a couple weeks ago, as I would have bought it. I like the idea of feeding more Omega-3s.
P.S. Organic chick feeds around here cost about $22 for a 50# bag.
I have had my 10 - 5,6 and 7 weeks old for a month and in that time they have eaten a 25 lb. bag of chick starter and are working on a 50 lb. bag of Purina Start And Grow.