The limiting factor for these boxes is the fill. I have access to plenty of pallets. Enough material for lining the boxes. Enough area to erect them. But each takes a cubic yard or more of fill. Believe me, that's a lot!
A truckload of weeds and trimmings looks like a cubic yard, but don't be fooled. It's mostly air. Once the material is transferred to a pallet box and stomped down, it's a lot less than you'd guess. Then give the material a few weeks to start decomposing, and the volume goes way down again.
This is how I'm using these old weeds. The stuff is rather woody, which means that it will be slow to decompose, is low in nitrogen, and will need fungi to break it down. Plus it is loaded with weed seed. With all this in mind, I'm loading a foot deep layer of the material into a pallet box. Then I add a 5 gallon bucket of dirt that I spread around over the layer. Next I sprinkle a couple shovelfuls of compost that I had previously inoculated with mushroom spore (I collect mushrooms from the county parks around here), then a 5 gallon bucket of manure (rabbit, chicken, or horse), then wet it with about 5 gallons of water or more. Repeat. Repeat. Climb in and tromp it down real good, then start all over again.
Using these old weeds means more work in filling the pallet box, but it will make a nice growing media. I've done it before so I know that it will work. This material will heat up. No surprise since its just a boxed compost pile. But that means that I can't plant into it immediately like I can the other "cold" pallet boxes. It will take 3-4 weeks for it to cool down then be topped with 3 inches of good garden soil.
So how much old weeds is needed to fill one box? I'm going to guess 18 standard full trashcans. Maybe 20. And that's just the first filling. I don't know how much volume it will lose as it goes through it's initial decomposing. The stuff is rather woody, so I'm not sure. Surprised to hear 20? Well so was I the first time I tried filling my first pallet grow box. It surely takes a lot of weeds to make a cubic yard of packed organic compost. And what also surprised me was that within two months the soil level will drop a full foot, and after 6 months the pallet box will only be half full. But the beauty of the system is that I get to utilize weeds, purchase zero fertilizer, get a lush crop, and end up with a half cubic yard of gorgeous "soil" that I can use to make more boxes or apply to garden rows that are rather sparse on soil.
I plan to grow chard, kale, and lettuce is these new boxes since I've had requests for those crops.
One more thing.......
I've already related how I line the boxes with something to help keep moisture in -- old tarp, cut open feed bags, old plastic sheeting. Up until now I use to use a staple gun to attach the liner to the pallets. But the plastic tends to pull through the staple during the filling/stomping process. So I'm trying something different. I'm taking old milk gallon jug tops, making a hole for a nail in the center of the top (with a drill or a soldering iron), then using a roofing nail to hammer the liner to the pallet.