A watched kettle and all that!
I think ours arrived at 20 weeks and were just short of their point of lay time. One started about a week later and then other a week after that. It's very frustrating - my daugther expected eggs that evening of their arrival.
It also depends on their breed but it's usally around 20-22 weeks. Be warned the first few eggs can be "odd" - shelless, misformed but don't worry.
One thing is for certain you will certainly here about it when they do lay for the first time - a shame mother hens don't tell their daugthers what to expect.