When should I apply for a Bridging Open Work Permit relative to my current permit?

You should apply for the BOWP before your existing work permit expires so you can keep working under maintained status while the new permit is processed. You can only apply once you have a permanent residence application that has passed IRCC's completeness check (for PNP applicants, you provide the acknowledgement-of-receipt letter). If you let your work permit lapse without applying, you may lose your authorization to work and may need to apply to restore your status, so timing the application early is important.