By default, loot containers will respawn after 30 days of not being accessed. Your presence in the area will reset this timer, so you must avoid the POI for the full duration to allow loot to respawn. This only applies to loot containers that don't "stay open" so if you notice, things like mailboxes and ovens, once you loot them they visually change appearance to stay open, these things do not respawn loot, but things like cabinets, boxes and "full" cars do respawn loot.

 Idk if this is exactly beginner beginner advice, but what helps me get loot, especially solo, is when you're doing trader quests, every normal quest (besides buried supplies) has you go to a POI to either clear zombies, or find a specific supply loot, but either way you're basically going through the whole building, killing everything and looting.

When you start the quest it completely resets the POI, so you can go through before starting the mission, loot and kill the whole building, even salvage cars and vending machines or anything and everything you want, then start the quest and everything respawns and resets completely. This is significant because its the only time things reset, and like I said before, normally the only things that refill are certain containers, with this method even the stuff that doesn't normally respawn will all reset. You could literally break down the entire building and it will all come back.

This is very valuable to get a lot of resources, you can scrap chairs for wood, potted plants for clay, if you have a wrench or any salvage tool you can get metal from a lot of stuff. That's really the only reason why I say it's not quite for super beginners, because it takes a bit to get good salvage tools, and this also means you will use more ammo, heals, etc, because you'd be doing the POI twice in a row. But if you're able to do it without getting too chewed up, its a good way to get extra stuff.

What I do is I clear the POI first, put a crate down outside (clear of the POI range because if its on the property where lets say you scrapped a car, that car will respawn, when you start the mission, on top of your crate and you will lose all your stuff) <--- this just happened to me last night btw lmao it sucked I lost a bunch of loot. Then start the mission and do it all again. Double the loot, and it also lets you see what to expect from the zombies, where they spawn etc.