Do you mean how old they have to be to lay, or how long after mating, or how long they take after they start to look like they're going to lay?
They can lay eggs very young, but it's not healthy for them before a year old. If you have a pair, there's no guarantee they will ever lay eggs, most will, but how long it takes will depend on their health and the season. Once a female starts acting broody, going in the nestbox a lot, eating calcium, ect. you need to watch her closely for signs of eggbinding.
General good nutrition is important. To get them in breeding condition, it's important they get plenty of fresh foods, such as greens, veggies, and soaked seed. The female should always have access to calcium, such as a mineral block or cuttlebone.
Like others are saying, there are always lots of mostly poorly bred budgies, pretty much all pet stores have them. Don't breed more unless you know what you are doing, know there will be homes for them, and are breeding only from nice healthy budgies (preferably budgies that are from a high quality breeder themselves).