I'm not sure, by using history I would say not long but america has done something unprecedented. They have made allies all over the place. I'm not sure how this is significant its just that its not like some unknown empire will arise and take out america while it has all those allies.
What is more likely is one of those allies eventually becomes so powerful that it, almost peacefully, takes americas title. I think america would remain an allie of whoever that was and this trend would continue for a long time. The days of conquering empires are probably over.
All powers of the world are known and monitored.
What nation do you think will be next?
I honestly think its australia for lack of a better option, reason being it is so young and unpopulated yet it is wealthy and has high living standards. It has the benefit of progressing while science already has a great depth of knowledge. It is on its way up while america is clearly on its way down. It is too easy to remember when america was a better place but australia improves in everyway all the time. Most other nations have already passed their use by dates, they are finished and there are no realistic big time goals in their futures. But australia stays small and clean and pure. It hasn't even got halfway up the hill let alone over it. It might take time but I think australia will be the next super power.
I'm not saying an americaVS australia war is imminent
I think the title will just be silently handed over by public opinion and both nations will remain allies.
The only other possible candidate I can think of is canada but australia are behind them and for some reason I see that as beneficial at this stage in time.
Kind of like the queens children, its like canada will become elderly while the us is still not finished being the queen and the young australia will be perfect for the position.
I can't think of any other nation to even recognise as being in the race...