You must do this within 5 months after your duties start date.
Even if someone else has helped you with your duties and may even be completing the declaration for you, it is your legal duty to make sure that the declaration is completed on time and the information entered is correct. If not, you may be fined.
You can only start your declaration of compliance when you receive your letter from us, as you'll need your letter code and PAYE reference. Although you have five months to complete your declaration, we recommend that you start as soon as possible after you've carried out your automatic enrolment duties.
If you have all the relevant information to hand it can take as little as 15 minutes to complete your declaration. To help you do this you should use the declaration checklist which shows you all of the information you will need and where you can get it from.
Once you have completed your declaration of compliance you still have ongoing duties towards your staff.