Depends where you live, what website or doctor you listen to. Most say at least a year and then to stay on it till age 2 before switching to 2%. Thanks to the pressure from the formula companies that come through with research money for doctors in exchange for pushing products it is hard to tell what is developmentally correct. Now in the US there is a push to only serve 1% to combat obesity.

Basically it comes down to you can probably do whatever you want and there will be some support somewhere.

I always based the switch on the child and how well they were eating. Poor eaters had milk at meals but formula morning and night for the extra nutrients.

I only serve 2% so if parents regardless of child's age want formula or homo they must supply it and I serve it as long as they supply it so the switch over is up to the parent and doctor.