The battery life span debate is like the "How long will my Tyres Last" and "How long will my chain last" debates - everyone has an opinion and different experiances.
I've got an 02 B12 and am now on my 2nd battery, replaced it last year. I ride to work everyday, but it's only a really short distance (12km round trip), but I'm too lazy to ride a push bike or walk.
B6 is right, there are a shyte load of factors that effect battery life, and from what I've been told extremes of temperature really hurt the life span, for example here in sunny downtown Wagga in summer 35-38 is about the average daytime temp getting down to maybe 20 overnight. In Winter though it's not uncommon for the temp to get as low as -5 over night and we're lucky if it reaches double figures during the day.