Think of your beacons like most other battery powered devices, for example your phone.
They use more power when actively used. If you actively monitor your beacon it will consume more power then when the crew watcher is turned off. In fact, if you turn the crew watcher off in the app, it will automatically put your beacon into sleep mode, consuming a lot less power.
So don't forget, if you are done using the product, make sure you have disabled the crew watcher in the app.
The battery is non replaceable for safety reasons. It will usually last for years. To estimate how long it will last for you we've made this spreadsheet. Just fill out how much you plan to go out on the left and find the estimates for your vessel on the right.