When I started to carry regularly in 2006, I used to stay with one for a couple years. Did so all till Sept of last year, from where I went with same pistol till April '14. Generally, reason for switch is looking for a new challenge, although going to hammer pistols was due to changing to AIWB.
Since April I use a pool of three, flipping between two of them while third gets something done. Admittedly, all of them are DA/SA or LEM.
Comparative performance eval doesn't really work if one stays with one gun.