When to have the future talk with my girlfriend ?

Too many unknowns. When people in the marriage/life partner age range (25+) talk about the future it's more immediate and they often have the maturity to know after a year or so if the relationship is a candidate for forever. So your age, life situation, income and all of that needs to be considered before even imaging that you can make any promises. When you're ready to be contemplating something longterm the "future talk" will have to include everything from where to live, how to live, work/life balance, children or not, religion, attitudes about money, etc. This is why solid couples usually spend a year or more just in discussion about this stuff. 

If you haven't realised you are life partners by month 8 then it should end. Naturally by this point you should also know if you want the same things in life otherwise you are both potentially wasting each others time.

Where's the relationship heading and if the person really plans to get married , or wants kids