As an usher I can tell you right away,that, even if you were hired by Shubert, you don't get to select the theatre they put you in. Generally speaking, if you're hired you start out as a sub. The length of time between doing that and having a permanent house that you always go to varies. I have friends who are ushers who came on board the same time I did and have a permanent spot, others have been working for far longer than I and are still substitute ushers. That part is a crapshoot.
As for shifts, it's a rotation. Some stay for the entire performance, others get to leave after the first half hour. They rotate and act as if there's a show on Mondays as a way to reset the rotation. In other words, If I'm on the late shift, staying the entire performance on a Saturday night, then the next Saturday, I'd be on the early shift leaving after the first half hour. Ushers do get breaks, but when and for how long is up to the head usher. The usher supervisor who dose payroll for ushers, decides who is there and if she needs a sub to come in etc. The length of the show comes into play too. But, I had a friend who ushered at, The Humans and they still got a break despite the show only being an hour and a half long.
You have to buy into the membership for Local 306, the union for ushers and pay dues every quarter. So, wth all that said, it's highly unlikely that you'd be able to usher Hello Dolly. Besides which, last I heard to my dismay, there's a hiring freeze right now for the Shuberts. Beauty of being an usher is that because it's all the same union I can work for both Shubert and Nederlander at the same time. Sorry for the long post but hope it's helpful.