Boy I had to do some research on this one, I had never heard of Warren breed before.
OK Isa Warren's are a production breed, suppose to be top layers of nice size brown eggs. Therefore my guess would be they will begin to produce at about 5 months. 20 weeks.
There combs will turn a bright red soon, once you see that the eggs will be coming soon, weaher, stress, available light and feed will all play a factor. If you have not started them on Layer feed, given the breed, I would do so now.