How much time does it take to become a good negotiator?

It depends on your Experience, your feedback, and your willing to be the best

No one knows for certain how much time it takes to become an expert negotiator, as it varies based on individuals circumstances. 

The learning curve is a life long journey, the bottom line is you need to practice it every day and pay attention to your negotiation process then reflect and adjust. 

Being a good negotiator is base on a combination of innate quality, training, and experience.  Depending on the amount of practice, dedication, and drive, you can become a good negotiator in a year.

I think negotiation skills are God Gifted ability

It is hard to define how long it takes to become an good negotiator, but there are some things you need to do to be prepared. These things are:

1. get some formal training, it is important you understand what is happening in negotiations and how to deal with it. There a many games played and you need to able to recognise these.
2. spend some time working with an experienced negotiator to learn what happens and how he deals with things
3. jump in and do it yourself. if you have learned from points 1 and 2 you should be confident enough to undertake negotiations. 

Time is crucial in negotiations but a good negotiator must have results within 48 hours from previous meeting for ample for an enough time for a good decision making from both parties.