When they can't get any more demographic information to sell to advertisers. So, never.
It's not free now, you're just exchanging not paying for giving a detailed consumer profile.
If you need something, it doesn't remain free. Once everyone starts using Google (like now), the demographic information is on a diminishing returns. Google now is competing with Apple on ads so they need to find some additional revenue streams. Google will start a premium paid service and eventually move much of their services into the paid premium service.
Similar to the tiered mobile data pricing, you will see an end to the "free" internet services.