If you use high quality food, they don't ever need vegetables. I give them to mine a couple times a week when they are available. They are a good source of
water, and they seem to like them. If you leave them in more than a day, they may start to get moldy, so I don't leave them in there. Whatever they don't eat in a day goes in the
compost bin. I also don't put too much vegetable matter in at a time because too much moisture will make their regular food get moldy and you definitely don't want that. I wouldn't over think it, there is lots of room for error with meal worms.