Main Post Office in most voivodeship capital cities and other big ones are open at least 8-20 Mon-Sun.
Other offices generally are open ~8-17 Mon-Fri but it also depends how big city is. In bigger cities it might be 7-20, and working Saturdays and even some Sundays like tax office in last weekends before the final date for submitting the tax declaration.
There is no lunch break in law, you have right to 15 min during the day. Lunch/brunch break etc is in employer hands and it is generally not calculated into working hours.