According to the New York Times, Boggess is contracted for 4 months and Lewis for 6 months which would make their last performances most likely September 13 and November 15, respectively. It's certainly possible for 1 or both of them to extend though.
I can't imagine Norm not extending, he seems to be having a ball, and his performance has generated the most coverage this show has gotten in years.
"Grease," the fourth revival of the season, is the worst show in the history of theater and represents an unparalleled assault on Western civilization and its values. - Michael Reidel
I'll be very surprised if Norm Lewis doesn't extend. To the best of my knowledge, Hugh Panaro doesn't have any plans to return to the show, so it's not like this is a case of "Peter Jöback handling the role while Hugh takes a break." Norm Lewis is great in the show, and I think six months is a bit short to be playing your dream role.
"Was uns befreit, das muss stärker sein als wir es sind." -Tanz der Vampire