Longer than they'll need it. I ended up blending scratch with the starter to make grower blend of 14% protien at 10 weeks to use up the first 50 lbs bag. We had 5 chicks and never bought grower blend. Blended scratch with layer feed too. Did'nt run out of starter until week 16 so blended with layer until they were 20 weeks old then striaght layer blend.
You can feed them the chick starter until point of lay, which for many breeds, is close to 20 weeks. I'd say it will come out OK for you.
Just store it in a snap lid tub to keep out pests and help keep it fresh.