My mealworms never liked it and after just a few hours, it started to stink up the room. I believe it said to replace every 24-48 hours, but I couldn't stand to have that stuff in the mealworm bin after 3-4 hours.
Instead, I have been using it in powder form for my dubia roaches and they love it. Not sure about the mealworms eating the powder form, though...but they still need some moisture, so it wouldn't work out that way. The dubias actually have a water bowl with sponge for water.
I use a variety of carrot, potato, apple, and other fruits/veggies. I've tried alfalfa hay at one point (since dubias LOVED it) but the mealworms didn't eat much of it.