Two feed stores around here provide competing information about this. One says starter until point of lay, and doesn't even stock the grower/finisher feeds. The other (the one I like the best for many reasons) says starter until 8 weeks, then grower/finisher, then layer when the hens start to lay. The employees there also say those are guidelines, not absolutes. If you've got feed of one kind left at the benchmark age, just finish using it, no biggie. Next bag, buy the next stage. If you run out a little sooner than the benchmark age, don't buy a whole bag of it, just start the next type of feed then.
And that's what I've done. My 9 chickens are quite healthy and happy.
Then I had to go get chicks and a couple of ducklings this spring, and then more chicks when those were put outside at 8 wks. Plus I have the layers, so I need THREE types of feed!