If you applied for permanent residency using the Express Entry system, you can apply for the BOWP as soon as you receive your Acknowledgment of Receipt (AoR) (automatically received on your EE account after submission of your application). You will need to add this letter as a supporting document for your BOWP application.
However, the BOWP will not be issued until your permanent residence application has been checked for completeness (R10 check) by Immigration, Refugees, and Citizenship Canada (IRCC), which is done within the first couple of months of the processing of your application.
If you applied on paper (outside the Express Entry system), you must wait until your receive, by mail or e-mail, your Acknowledgment of Receipt (AoR). This AoR will not be issued until IRCC assesses that your application passes the eligibility check. You will need to add this letter as a supporting document for your BOWP application.