Let's get this in perspective, In the old days memory was small the battery backup would only be enabled during power down, it would depend on many factors i.e. size of memory, type, battery capacity etc. during power up the battery drain is almost non-existent, so in theory the battery would last somewhere in the region of 10 years, as volatile memory has increased, & hardware platform became smaller the battery life (when powered down) has become shorter, most systems have a large capacitor so when replacing the battery it will hold the memory for about 10 - 20 minutes so it can be replaced with power off.
Any that copy to NVRam will obviously require higher capacity but the amount of times it is likely to be powered down in normal circumstances it should last, in saying that, like any battery backed system with non-rechargeable batteries will last for a shorter time if the system is powered down frequently.
I worked at a plant where I replaced the backup batteries on over 100 PLC's & HMI's of various makes i.e. AB SLC, Siemens, Omron, IMO, Mitsubishi Koyo & a few others (I know a lot of different makes I did over time move many to Mitsubishi), A PPM schedule was in place to replace all batteries, not once did I power down any system all were replaced live using compatible batteries in most this goes against the mfgr's recommendations but never had a failure.
this was over a period of 16 years so must have replaced over 1600 batteries, only loss of program was when an engineer removed the PSU of an S7 400 system (the batteries are located in the PSU & not in the CPU + one other Mitsubishi PLC that nobody even knew existed, this had been running for nearly 19 years before the battery failed & the system had a power outage.
I had one Mitsubishi A1 processor where it was PW protected, this was replaced with a Q series, I removed the battery assuming it would lose it's memory within about 20 minutes, 2 days later it still retained it as it did 4 days later, went back 9 days later it had finally lost the program.
I currently have an FX2N that must be at least 12 years old originally used for testing, the battery has never been replaced, been powered down for over 5 years & last week I powered it up for a friend to borrow & it still retained it's data.
Many now have battery-less memory I know that some use what they call a super capacitor that charges up when under mains power, these are often used in small cameras & can power the camera for about 2-3 hours.
Getting back to the OP's question, as others have stated, I suggest it is a get out of jail card, but get a correct replacement.