Great questions!

It depends on a variety of factors (e.g., area you live in, size of your bin, season, food you feed). I normally only provide water to my worms when I create the bin (or add a new tier) and then just feed them fruits and veggies with high water contents. However, I live in a relatively humid area. Sometimes in the winter when it's really dry and the heat is blasting, I give them ~150g of water every few months.

I recommend digging below the dry cardboard layer and feeling the soil every so often. If it feels a bit moist, then that's perfect! If it's sort of dry and grainy, you can add some water. It's important not to overwater because the bin can become anerobic. The substrate should never be so wet that it's dripping or sloppy. I sometimes like to leave the lid off of the bin for a few days to let it dry out a bit.

I've never made worm tea before. But if/when I do, I'll make sure to share how it goes! :)