How much capacity do I need?
5L → 15L: summer day hikes, ½-day hikes.
15L → 30L: 3-season day hikes.
28L → 45L: winter day hikes, summer overnights, hut-to-hut.
50L → 70L: 3-season backpacking (4-7 days), winter backpacking (2-3 days), thru-hiking.
75L → 95L: backpacking with young children, winter/expedition backpacking, professional guide work.
By the way, hut-to-hut hiking means any backcountry hiking that does not require the Big Four, because you will be sleeping in roofed accommodation every night. Examples include the Camino (though some routes have little or no backcountry sections), the Sunshine Coast Trail, or the Tour du Mont Blanc. The Bruce Trail is a strange example that stands on its own; consult the Bruce Trail Reference Guide for more information.
These guidelines are also not gospel. If you’re an ultralight fanatic, you don’t need as much capacity. Easy day hikes in the frontcountry don’t require this much capacity. If you have older, cheaper equipment, you may need a lot more capacity.