I've only had 3 phones in my life. From 10 - 14 a nokia phone for school, 15 - 17 a Blackberry Curve and now 17 hopefully to 19 i'll have my S4. Maybe in 2015 i'll get the next Galaxy note for myself for my 19th Bday.
Like a year and a half...I think it's too much but no one of my mobiles have satisfacted me. I want to buy an iPhone but my parents say it's too much expensive!
Up until now I've had my iPhone 4 from 3 years ago. Going to upgrade soon.
I only need to change if it starts slowing down, running out of memory, etc. It still runs fine with ios7, but new apps coming out are designed for newer gens.
my current galaxy S2 will complete its 3 years in June!! but it was extended because i dint really feel a need to change it, it still works perfectly and is properly updated.(and well i had been failing my exams and dint want to buy a new phone till a cleared them all this july! )
before this i changed my 2-3 phones within 1.5-2 years
)