Well, when they are around 20 weeks, give or take a few weeks. Some breeds start laying quicker than others. I've had some hybrid layers start as early as 17 weeks, and then I've had some large breeds, like Marans, start at around 25 weeks.
Some of the signs you'll notice that mean egg laying will start soon are, the hen's will "squat" when you reach out to pet them. That's a sign of maturity, meaning they think you are their rooster and are submitting to you. Also, their combs and wattles get redder. They may fuss around the nesting boxes for a while too, without laying an egg.