The lengthiest songs from cast recordings I have are:
Final Sequence (Sweeney Todd OBC- 14:10)
Simple (Anyone Can Whistle OBC- 12:57)
Prologue: Into the Woods (ITW OBC 11:5
The Final Sequence from Sweeney and the opening from ITW are comprised of several smaller songs, so I don't think those would count. Simple, however, is fairly musically consistent.
"...everyone finally shut up, and the audience could enjoy the beginning of the Anatevka Pogram in peace."