Same Time, Next year officially took 3 weeks.
Tribute, the same producer's (Morton Gottlieb) next play, paid off opening night. It had a huge star (Jack Lemmon) and had made money during the out of town tryout.
In both cases, and probably a few more, they were overcapitalized and had reserve funds that were never needed. Those were the first part of the recoupment.
The original Boys in the Band paid off in 10 days. It too had a large reserve and a shortened rehearsal period since they had all done a workshop of the play earlier.
David Merrick had a history of keeping shows out of town to try to make back some money before New York. Stop the World and Roar of the Greasepaint were both financial hits before they opened; I think Oliver made a large portion back on a lengthy West Coast tryout, and even Pickwick which was a 2 month flop, made about 50% of its investment back.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher