I visited in June San Francisco for 1 month but in my passport is printed a 3 months stay, now i will visit again on December with the same esta for 2 weeks. In short, how's this 3 months/year stay applied for multiple visits per year?
Please note that your ESTA authorization serves to allow you to board your flight. Your actual admission to the United States is under the terms of the Visa Waiver Program (VWP). ESTA is required for most of those who travel under the VWP, so the two are often confused.
The VWP allows 90 days per visit, not per year. Travelers who have frequent lengthy visits may arouse the suspicions of border officers, as they may appear to be abusing the VWP. The officer may deny admission after scrutinizing the details of the trip.
The 90-day limit is based on the actual days of presence in the United States, not on the period for which you are admitted by the officer as shown by the passport stamp.
You have not even come close to exceeding your allowed stay, nor does your planned travel come close to it, so you will be fine.