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The problem is that as soon as you're gonna stay the night in a tent you're adding like >30l of bulk right there for sleeping gear so there's no huge difference between a 2-day trip and an 8-day trip other than food, all else (weather etc) being equal.
I was out for a week with my 50l SWD Long Haul. About 30 lbs. I was pretty happy with that weight, everything fit no problem, no need to cram or overthink packing. It was summer though so I got away with fairly light clothing and I strap a lot on the outside. Rain/wind shell, crocs, tent. I even carried a bit too much food.
That said I don't know if those 50l account for the outside pockets. I could fit all of my food (sans trail mix) in the large pocket on the back, which was pretty fucking great.
Drybags to put tent and sleeping bag in and strap on the outside might be an idea. That may interfere with the rain cover (which never works well anyway), so I'd recommend just buying a waterproof pack that doesn't absorb much water. Like mine, which is made of X-pac which just sheds water. I still don't know if that material will last long however but not having to worry about rain is fantastic. Otherwise just use a trashbag for a liner.
I can't call myself a pro or anything but I've definitely improved after the few trips I've done. My first backpack was a 65l Everest I got because it was heavily discounted. It weighs 3x (but cost less than 1/3) of what the SWD one does. The Everest is no more comfortable either, and not waterproof.
Looking back, I filled up the Everest with so much garbage, packed like a retard and my gear was bulky and heavy (as beginner stuff is) but maybe those are mistakes everyone needs to start out making.
I'll say 60+ liters unless they can pack smartly and invest in shit like a small sleeping mat, stove etc.
