Tell after how much time i will need to replace battery of my iphone?

Please tell after how much time i will need to replace battery of my iphone?

Possibly after 3-4 years

It depends; it's really your choice! But often, after one year, your battery would've deteriorated already and a full charge wouldn't last as long as when you just bought it. I think one year is fine but it's ultimately your choice; some never change at all!

Depends on your usage