I have had mine since 1 years. When do you change, when you see the 'red mark' on the tyre or after a couple of years in use?

There are tread wear indicators on the tyres, which are rubber blocks across the width of the tread, when the tread is low enough these form a strip of rubber right across the tread of the tyre.

Tyre life depends on how you use them. Regular inspection should tell you when to replace them. Don't be tempted to think that you have only had the tyres a short while so they must be ok.