Usually every 3-4 years or depending on how long my phone lasts.
I mean I only replaced my Motorola Razr (v3, not the new android based version) because the front screen broke and the battery life went from like 4 days to 6 hours. Similarly my Samsung Wave got replaced because battery life started to suck (down from 5 days to 2 days) and it couldn't handle browsing the web without crashing the whole phone. And in another 2-3 years when my Samsung Note 3 starts to die I'll replace it with something. (Possibly the Samsung Note 6/7 if they keep going and aren't stupid about it... I mean keep the microSD slot and removable backplate/battery.)