That is a really tough question to answer as it will depend on how much you use it, what advanced functions like the digital compass you use regularly, is it connected to a wireless transmitter, what water temperatures do you dive in, the actual chemical make up of the battery itself? All of these variables and more will affect the life of your dive computer battery.
The average mid range dive computer without a fancy colour screen and an average diver doing maybe 50 dives a year might expect to have to replace their battery coming up towards the second year. More or less use will obviously affect the life but most manufacturers state a two year life. With the cost of battery kits being so little and the vast majority of computers allowing users to change the batteries themselves it makes much more sense to swap them more regularly than that for peace of mind though.
Batteries can be tricky little buggers to predict and whilst many computers show a power level indicator on the display to warn you when they are getting low they are infallible. Not all batteries were created equal and as with much in life you get what you pay for. The manufacturers go to extra lengths to ensure the batteries they put in their dive computers meet their exact requirements and specifications but unbranded and cheaper variants are generally only there or thereabouts in terms of both power and quite often size. They might even use a different (cheaper) chemical process to generate the electricity which means they burn themselves out quicker. It is always a good idea to opt for a genuine battery kit for that reason.
Temperature can also significantly affect battery performance and is the cause of the sudden computer failure after jumping into the water after a longer period in storage. Batteries use a chemical reaction to create electricity. Cool batteries perform at their optimum but start raising or lowering the temperature and it affects the chemical reaction and in our case it inhibits the process, producing less power which lowers the voltage output resulting in the computer deciding it isn't receiving enough power and switches off. Confused and annoyed, you end the dive, get out of the water, de-kit and by the time you get back to look at it the battery has warmed up again and the computer happily fires up. That is the main reason most dive centres recommend computer batteries are replaced annually.
Finally, the amount of computing your dive computer has do to will cause the battery to deplete faster. Connecting to your PC or smart device regularly will run extra hardware and processes to transfer the data, digital compasses require lots of screen updates and processing power, screen brightness uses more power. The more technology you have running means either you need a larger power supply or a shorter battery life so if you are trying to stretch out your battery changes just think about what you have turned on.
Don't fancy doing the battery change yourself or got a watch style computer that isn't user changeable? No problem, we offer a drop off and collect or ship and ship back battery change service for pretty much all brands of dive computer.