No clock. What you bought stays on your account and you can start whenever you like.

Each purchase gives you three attempts at that exam, and buying the same exam again adds three more rather than replacing what you already have. You can see what is left under My Exams in your account.

One thing worth knowing. Once you open the review of a completed attempt there is a window on how long that review stays open, so it is worth sitting down to do the review properly rather than opening it for a quick look.