When should I apply to extend my Polish single permit before it expires?

Because the single permit is temporary, you need to apply for a new one before the current permit runs out if you intend to keep living and working in Poland. Submitting on time is what allows your stay to remain lawful while the authorities consider the new application.

Applying late, or letting your permit lapse, can interrupt your legal status and your right to work, which may have knock-on effects for any future application, including for long-term residence. As a rule of thumb, treat the renewal as something to start weeks ahead of the deadline rather than at the last moment.