Does keeping an item in the box help respawn?
I keep a piece of paper or grass in all my loot boxes and even containers in buildings near my base.
It seems to work, or maybe I am just fooling myself.

While doing some testing in single player I came across an answer to this. I was testing to see how often I could find an engine in working stiff tools crates. So I setup up 10 in front of me and did the initial looting. I soon discovered that when I fast fwd'd the days nothing would respawn with them empty and me near them(5ish blocks).  The loot respawn timer only starts when players are outside a specific radius and resets if that zone is entered. So I tried the leave something in them trick and voila.  Evidently leaving an item in the crate made the 'player proximity reset loot respawn timer' (a mouthful I know) effect null and void.

Now, when I fast fwd'd the days I went past the 5 day respawn date so it may help to play it safe in a similar manner. Or, just keep track of exactly when you loot so as not to loot again too early and resetting the timer.  I am unsure of the exact proximity that affects the respawn timer but a brief search of the 7DTD forums shows that it may be as small as an 8 block radius.  I think I'll test the proximity in the next few days when I get time.

Lastly, when you're sure you can loot the crates again after it has been enough time for the boxes to reset with loot.  You simply search the box, remove the item(s), and back out of the box.  After that it should take the box 1-2 seconds to reset to untouched status for you to reloot.

Hopefully this helps all of you that have already purchased crates and put them in high traffic areas.  Good luck out there and happy zombie...or player killing.

Just checked if the loot respawns with the player near and nothing in the crate.  With the crate being block 0, wood crates as 1-7, and me as block 8, or further away, I was able to have the loot respawn like normal without putting anything in the crate.  So, it seams as though there is a radius of 7 blocks around the crates that act as the proximity zone.  Also, I did this all on level ground and not above or below the crate.  I did this in SP by speeding up the time in the debug menu.  I tried using the debug menu to skip days directly but that did nothing leaving the crate still empty regardless of distance.

Diagram    [Crate][1][2][3][4][5][6][7][Me]

Thank you!!! This is good to know. I appreciate all the effort you put into this,

Update:  I was away for 11 days.  It worked and I was able to loot the crates/boxes again.  The number of days is probably less, but the process works.   

Incidently, I've settled on 8 days between looting the crates in my "crate cave"  Working well.  I added a couple sedans down there too, but I have no idea if they are loot-able as they were empty when I placed them.  Sure could use an engine or a battery occasionally. 

The cars you get for zcoins are not lootable. Get the car parts crate if you want batteries and engines.
Qikstart, you need not loot crates. Me and John love doing it.