The battery life of an average tank sensor should be ~16 months given a good set of Alkaline Non-Rechargeable AAA batteries.
If your batteries are draining within a short space of time - say a few weeks or a month - there are a few things you can check. One relates to a rare bug seen on version 3.0 tank sensors, and the other relates to the type of battery you're using.
1) Log into your account and click on "devices". Check if your tank sensor is running "version 3.0" firmware. If it is, click on it, click on "what's new" and click "update device".
2) Check that you've not inserted rechargeable batteries; our units have a low current draw which means that rechargeable batteries (naturally having a shorter life than non-rechargeables) will not be as cost-effective as you might think! Good alkaline non-rechargeable batteries (Duracell or Maxell ideally) will last a substantial amount of time in comparison.