The cooler you keep them, the longer they will last. If you have a wine fridge that can maintain like 55f degrees, they will last a couple months. At room temperature they will last around 2 weeks.
Hard to know for sure how much he will eat. I recommend starting with a small quantity to make sure he likes them. Some don't have a taste for them. But, it's not uncommon for them to go through 40 or 50 a day. But it of course could be lower too.
I like to always feed multiple bugs, so I never really let my dragons fill up on one specific bug.
-Brandon