To set how far in advance clients are able to book online you first go to your Settings tab in the left hand side menu.
Under the Your Settings title click on Booking Settings. This will open up your booking settings options on the right hand side of your screen.
At the top of the page you'll see a text box where you can fill in the amount of days in advance that you'd like your clients to be able to book online. See example below:
Hit Save and it's all set. You can go in here and change this in the future if needed.