How many year can we work as developer ?

Your question is subjective. Retiring from coding really depends on the person. I have over 11 years of experience on coding and up until now I still write codes because that's my passion and that's what motivates me.

 

I have been offered alot of managerial skills, even my current company offers me a managerial position but I didn't accept it because I still enjoy coding. Also, I'm not after the position, If higher position unmotivates me then I'd rather stick to what I'm doing now.

 

My motto is work for the position which gives you happiness and motivation. Once you are good at what you do, salary will follow. It's not necessary to become a Manager to have a great salary. I know alot of developers who are earning much more than a Manager.