Batteries? Well, it depends. Sometimes they last 2 months, other times they last 7 months. Maybe it depends on the brand and the quality of the batteries?
Anyways, your only option really is to pause it. When you pause your Tamagotchi it stops the growing process and everything else right where it was when you paused it. When you unpaused it, it will be just fine, and it will start back from how it was when you last played with it.