Short answer is that it will continue until it is no longer profitable. At this point the series has changed and evolved so much from it's origins decades ago that I can't see them laying it down when if/when it becomes irrelevant. It continue to change to adapt to the market share. Only thing that will stop the series is when they can't keep the lights on to make the next installment.

I read somewhere in 2003 that SE had a contract to make up to FF15, but I have no idea if that is actually true.