You can start measuring the first day, or at your earliest convenience after you quit your hormonal contraception (birth control). Please note that it can take a while for your first ovulation to occur, and you will be given only Red days for Prevent, and brown days for Plan, until it occurs. Ovulation is in general inhibited while using hormonal contraception (birth control), so it may take a while for your cycle to become regular again. With each ovulatory cycle, you will get more and more green days, but in the beginning, you will experience many red/brown days.
It is highly individual how long it can take for the first ovulation to occur, it can be from a few weeks to a few months. With this in mind, the better you measure, the quicker our smart algorithm will be able to detect when you have ovulated. In the first cycle, you do not need to use LH tests as a Prevent user, as it will be difficult to tell when you might ovulate. As your cycle has been on hold for a long time, you may experience a lot of red days during your first months with Natural Cycles. This is a safety precaution by the app when in Prevent plan.
For a Plan user, you should take LH tests when/if the app suggests you to, otherwise, it's still just about measuring often.