I've been to a bunch. Duration depends on where they are. Parties at the Marriot (NEVER GONNA DANCE, FIDDLER ON THE ROOF, LA CAGE AUX FOLLES) and at the restaurant under the 59th St. Bridge - I can't place the name - (GYPSY, BROOKLYN, SWEET CHARITY) - tend to get a out a lot earlier - maybe 11:30-12:30 range, where as parties at Tavern on the Green (LITTLE SHOP OF HORRORS, WICKED, DEMOCRACY, etc) last a pretty long time - pretty much until people leave, as well as The Copacabana (THE BOY FROM OZ, DIRTY ROTTEN SCOUNDRELS) - which run pretty late. Roseland Ballroom also throws nice parties that last a while - especially for HAIRSPRAY and SPAMALOT.
"If you are going to do something, do it well. And leave something witchy."
-Charlie Manson