The Assessment starts on the day of your first trade in your connected account, so you can begin whenever you are ready.
There is no deadline. You need a minimum of 35 active trading days to pass, and those days do not have to be consecutive, so you trade at your own pace.
Once your account is connected you can review the dashboard and check your broker setup before you place your first trade. Tracking begins with that first trade.
If a breach ends an attempt, you can reset at no cost and start again straight away, so there is never a wait for a start date.