Some people like to bring as little as possible, some people want to bring some extra luxury. The packing volume you need depends on who you are and where you are going but there are some general pointers.
10-30L Daytrips without a tent.
30-50L Day- or overnight trips in a hut or with tent.
60-75L Multiday trekking with tent, food, change of clothes, kitchen gear.
90L+ Long treks and excursions where you need to be self-sufficient for 7+ days.