Rafa's in a strong position to be number 1 for quite a while, if he wins the US Open.
There's no way that Roger is going to defend the 4,000+ points he won up to the the clay court season, and Nadal ''only'' has something like 2,300 points to defend, and may even gain on that.
He could be 2,000-4,000 points clear of Federer before the 2018 clay court season begins, so wouldn't even need to repeat his clay feats of this year.
If Novak and/or Andy are rejuvenated after their break, they still wouldn't have enough time or points to challenge for number 1 until, at least, Wimbledon.
The only other possible contender would be Zverev, if he gains some momentum and strong results in early 2018. Other than that, Rafa (barring injury) should hold the number 1 spot for at least 9-10 months - should he win this US Open.