There is no set formula, although the Feed companies typically put some options on the back of the bag. You can keep them on "starter" all the way to point of lay. You may wish to switch them to Grower or use Flock Raiser at 8 weeks or so. As long as you don't feed them straight layer feed all the time, they'll do well. The layer feed has added calcium for the layer to make egg shells.
I usually stop buying starter once the chicks are big enough to leave the comforts of our house and move to the coop brooder...3-6 weeks depending on weather. Once they go out I start feeding the entire flock a multi-purpose feed like Flockraiser. I have also used a higher protein turkey grower and cut it with scratch to bring the protein to something around 20-22%...usually 2 parts grower to 1 part scratch. My existing laying flock seems to enjoy the change for a few months too. Then about now, when my pullets are around 16 weeks old, I let the flockraiser run out and start purchasing lay pellets for the rest of the year. I will generally purchase a bag of scratch to feed sparingly if we have a really cold spell in the winter, just for a bit of entertainment and variety during those cold, dark winter days. But they stay on layer until I bring the chicks out again in the spring.
