no hard rules... depends on:

1) the bunga on your tyre (use the 20 cent rules)
2) when you feel your car losing grip during rainy days
3) Get harden very fast if you often park your car under the sun

Usual 3 years as rule of thumb.. if your tyre had harden but still got bunga, better get it change as it can get burst or losing grip on wet surface..

If wanna preserve you tyre well:
1) regular rotating
2) park under roof
3) ensure adequate tyre pressure
4) wash your tyres
5) avoid driving like Schumacher style on the road

my 2 rupiah..