Federer will at least be number 1 until the end of the Olympics, which means 4 more weeks, which means at least 289 weeks, that is more than Sampras's 286 weeks.
Djokovic can take the number 1 back after the Olympics : actually he will even take it if he just does the same result as Fed in the Olympics,
if it's at least a quarterfinal (because Fed will replace his 90 points from Doha whereas for Djokovic it will be a completely new result).
BUT after the Olympics, Djokovic will lose many points ... and as you will see below,
there's a very big chance that Fed is number 1 during the US Open and until Basel-Bercy (the points from Bercy and last WTF being taken out at that moment)... and that he reaches more than 300 weeks at number 1.
Now as far as the rankings are concerned, what will be important will be the Race for the end-of-year number 1, which would also mean for Fed potentially equalize Sampras's 6 end-of-year number 1s.
At the moment for the end-of-year number 1 Fed, Djokovic and Nadal are nearly equal :
Federer 7085 ahead of Djokovic and Nadal 6840,
which means the best from now on (and especially the winner of the US Open and/or the WTF 
) will be number 1 in the end of the year.
Before that, here are the ranking points until Shanghai when you take off the points from last year :
after Wimbledon :
Federer 11075
Djokovic 11000
Nadal 8905
after Canada open (won by Djokovic last year then he would drop 1000 points) :
Federer 10985
Djokovic 10000
Nadal 8895
after Cinci for the US Open seeds (Djokovic was finalist and will lose 600 points) :
Federer 10805
Djokovic 9400
Nadal 8715
after the US Open (Djokovic will have 2685 points to take back to Fed until then !!) :
Federer 10085
Nadal 7515
Djokovic 7400
after Shanghai :
Federer 10085
Djokovic 7400
Nadal 6995