How often should I replace the glue layers on the bed?
Depends on the glue and on your tolerance for messy undersides on your prints. It's fairly common for some of the glue to come off with the print. Or you may have marks from scrapers or rafts. Do you want to touch up that spot and have some artifacts on the bottom of the next print, or clean and redo the bed to get everything flat?

Gluestick is pretty easy to wash and reapply. It can also be freshened up with a gentle spray of water, smeared flat, and redried, or more gluestick added on top. It will really come down to your preferred workflow.