it depends on how good the battery is when you install it.
it also depends on how good the internal capacitor is on the plc should the battery fail. temperature will also change things.
before you turn the power off, you need to get a good backup of the program in the plc ! the plc program is the master, the hard drive copy is just in case,
bubba loves to make changes and not tell people. Learned that the hard way.
we changed the battery every year in our systems as part of our operating procedure.
james