I've got nearly 20 years old lithium batteries still at the voltage marked on them.(Used to save data like on PCs)
It always depends of the mobo also. I have one that drains batteries in about a week when left unplugged. I don't know the average time for the PC battery but I know that the battery wasn't dead on my 1996 jetway j-656(?) motherboard a few months ago.
Oh, and I've got a 1982 tuner with a CR2032(Same as most computers) that still works.