After a sports concussion, after how many months before you are back to normal, and consider yourself as having had zero concussions?

Never. Injury from concussion is cumulative over time. It's more dangerous to have them close together, but the risk never goes down to the level of folks who never had one before, even with years between them. Always consider the importance of high risk activities if you have a history. If you've had more than 2 in a season you may consider not participating. Talk to your doc if you have questions.
Concussion. Concussion history never goes away. Symptoms may go away. Having a concussion puts you at a higher & lower threshold risk of another if you continue high risk activities. This is a highly debated topic in the medical community. No matter the opinion, people with interest and training in this topic will be able to advice you when is ok to return to duty with the least risk for you. No guaranties.
Depends. The effects of concussions can vary and be unpredictable. It depends on the severity of the injury. The best thing is to give your brain complete rest. The more strain you place on your brain while it is attempting to heal, the more likely it is to slow down the healing process. You should not return to your regular physical activities until your are completely symptom free and back to normal.