No one has mentioned CAROLINE, OR CHANGE, which is an ensemble piece, but Caroline is almost always on stage and sings a good part of the score.
I feel like Eva Peron, Fanny Brice, Caroline Thibodeaux and the female lead from THE APPLE TREE are all massive roles in terms of stage time. Momma Rose is of course the mother of musical theater roles, even if she is not on stage all the time, she does sing so many songs in the score, I feel Reno Sweeney is a similar role too.
I agree with Margaret Johnson as well.
To go with male lead roles, there's Guido Contini from NINE which I've heard is one of the most demanding roles in terms of stage time and the vocals.
And how about the female lead in GREY GARDENS who has to play Big Edie in ACT I and Little Edie in ACT II?
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"