Generally most won't take more than 7 months unless they're something like a Jersey Giant or something else slower to mature. But there can be freak birds in any breed that take forever it feels like.
Ensuring that they're on layer feed and are getting 14 hours of light helps but there's nothing you can do to rush them into it. Some can start early, same as some can start late. I had one start at 4 1/2 months, tiny, perfect little eggs, every day. Crazy!
Look for squatting behavior, reddening of the comb, and getting bigger/fluffier in the rear end (as if it "drops" in comparison to younger non laying birds). They'll also start checking out the nesting area, though sometimes it's just to kick the bedding out.
Individuals can vary though. I have a Silver Spangled Hamburg who's been squatting for a month. She's only 5 months old, but she's throwing signs that she will, but her breed usually won't lay that young. She's supposed to be more of a later bloomer so she must be just teasing.
I had a duck due to lay, she gave no indication at all until 2 days before when she made a nest. 2 days later, and egg was in it. And every day since. She's been more reliable than any of my chickens!