I bought my car at 4 months old and at 2000 miles. After a few weeks of owning it I got the battery warning so I just used the second key and have never had a problem. Clearly there's no consistency in the original batteries
 
Just need to ask when I get a new battery is there anything I have to bear in mind

Nope, Open key cover, pop the out battery out, put the new one in and replace the cover. Should work without needing reprogrammed etc (mine did anyway!)
 
3.5y and message appears first time.
Changed in both keys as it is "cost less" and Murphy's laws are still working. :D