I have three girls, all hatched late March 2006. Two of them are RIRs and the other is an EE. The EE was the first to lay at 20 weeks, 5 days. The first RIR to lay did so at 23 weeks, 6 days. The second RIR didn't lay until 27 weeks exactly. Since your girls were hatched earlier in the year, it's likely that they'll lay a tad earlier than mine who came into maturity in the fall when the days were already getting shorter.
You'll begin to see some signs that they're close to laying. You'll notice that their comb and wattles are getting red. They may also do a funny little squat in the days before they lay and may also begin checking out the nest boxes.
I switched my girls to lay feed with calcium at 17 weeks, which was a little earlier than I would have liked to have done, but had run out of chick feed (no grower feed available in my area) and didn't want to end up with a bunch of feed I couldn't use. It would be reasonable to aim for about 20 weeks, but if you begin seeing signs of impending laying activity, you can begin switching over. It was suggested to me that I add 1/4 lay feed to their old feed for 4 - 5 days, then switch to half and half, then 3/4 and 1/4 until I eventually reach full lay feed.
Calcium can be given "free choice" in a separate feeder.
