I've been wondering how long Game Boy Advance (GBA) cartridge batteries actually last. I have Super Mario Bros. Deluxe for the GBA and it uses a battery to make a save file. Can someone help me find out how long one of these save file batteries last?
The CR1616 battery in that cartridge is said to last about 10-20 years from manufacture. This means that, if you have the original battery, there's about a 50% chance it is dead right now, or will be very soon. This information was obtained from figuring out the type of battery found in that cartridge and this information, which includes that battery under GB/GBC specs (but presumably, GBA should be similar in characteristics). You should probably consider replacing the battery soon if you intend to save. Note that removing the battery will immediately erase/lose your saves without specialized equipment to either keep a charge or a way to keep the data from degrading (e.g. using extreme cold to slow the decay rate). You can also back up your cartridge data with a reader like Submodule GB01 or others.
In addition to phyrfox's great answer, it should also be noted that games which have a real-time clock tend to drain their battery at a faster rate (due to the energy consumed to run the clock) than games which just use battery-backed RAM for saving.
However, some games such as Pokémon Emerald have a battery to run their real-time clock but save into flash memory. When the battery dies on these games, your save is not erased but the clock functionality won’t keep time with the GBA off until the battery is replaced.
This is just my own experience, but I have kept a handful of original GBA cartridges (as well as GB and GBC cartridges) along with my own original GBA SP that I've owned since it came out. Picked the games up mostly at a GameStop circa 2005. They have lived in their own little boxes inside a slightly larger box the whole time as, though I rarely use them, they're pretty dear to me.
As far as actual GBA carts, Mario 3, Zelda ALTTP/4 swords, Final Fantasy 4/5/6, and Advance Wars all still load and save data, and most still have saves that are close to two decades old. I'm sure this is due to them being kept very clean and mostly untouched, but in any case, I actually just took a look at all of them, and even the GB/GBC carts (ZELDA oracle of ages/seasons) still have old saves on them that load properly.
As is the case with anything, YMMV.