Usually worms wont totally leave a worm bin if there is food still there , but at the point they move into an upper tray then you know. If there is just a few worms in the bottom tray its probably ready, since you are feeding only the top tray. As for coffee grounds I try not to add a lot to a bin , but I do like add it only as part of the bedding.

You may be able to go by the length of time the tray has been operating, if you have a well established worm bin (going at least 6 months or so) with a good population of worms. If you swap out trays approximately once a month, the third tray at the end of three months will probably be ready to use.

Also, assuming that you are adding other food scraps to the bin and not just coffee grounds, you can observe when the other kinds of scraps are thoroughly broken down - at that time, the coffee grounds will also be processed enough for the vermicompost to be usable. The bedding materials will also look broken down.

Coffee grounds are a bit slower to break down than many other kinds of food scraps. They are tough and they contain compounds like tannins, so other microorganisms have to break those down for a while before the worms can digest the nutrients in the coffee grounds. That's why you don't see worms "cluster" around coffee grounds the way they do around sugary, watery treats like melon rind and pumpkin. But coffee grounds have a lot of nitrogen and make the microherd in the bin, and ultimately the worms, very fat and happy.

My biggest caution with coffee grounds is to try and mix them around into the bedding when they're first added, as clumps of coffee grounds can heat up briefly when they're first put in - the high nitrogen levels feed thermophilic bacteria that generate heat - but this effect doesn't last very long and won't hurt the worms as they'll just move away from the hot spots. It could potentially cause a problem if you added a really big load of coffee grounds all at once and didn't have other trays going that the worms could move into.

Thank you very much for the comments, y'all.

I actually moved an established bedding with worms into the tray system a months or so ago. They are set up in the first tray. I have not added another tray yet. Shredded newspaper are mostly gone and vegetable scraps disappear very fast.

Because of a large amount of coffee grounds, I cannot tell how much casting is actually there. I used to use much less coffee grounds, and it was easier to tell when to dump out the bin to harvest the casting when I was using a single bin system. I suppose I can simply use the casting and coffee ground mixture as fertilizer.

I am going to start another tray with much less coffee grounds and see what happens.

Do you cut up vegetable scraps into tiny pieces with a blender or food processor before adding them to the bedding? I see potato skins are hanging around for a long time.

My worm tray is still outside (right next to my kitchen door.) The worms seem to be slowing down with the cooler temperature. Do you recommend moving them inside? I am in Zone 7a. I am expecting the first frost/freeze this Sunday.

For people that use a multiple tray system is easy to tell when a tray is ready to harvest . If you are using 4 or more trays by the time the top tray gets to the bottom you can assume its ready , though usually I get busy and dont get it done when it could be.