If you look.. the first 4 option's are roughly equal...I'm guessing as after 4 years the batteries are usually pretty degraded and people fancy a change anyway.
I answered three to four years but I bought a new phone a couple of day's ago so if I voted again I'd vote under 1 year...
So looking at the figures as a whole, it suggests the majority People keep them 3 to 4 years, but how they vote depends where they currently are in that time.
For example the number's drop right off at 4 years or more.