A long running theater production of a musical in London or New York often has shows most nights and periodic matinees which when added to continuous rehearsals would add up to a staggering working week (I'm thinking about shows like The Lion King, We Will Rock You or Wicked). Muscles and voices need to be rested!
The typical (non-lead) cast member may be contracted to appear in all the performances. An understudy is provided for the main character(s) only. Additionally, the principal understudy may be contracted to appear in all the performances as well, taking the lead part during all matinees and one evening show per week (not the weekend), and performing in the chorus the remainder of the shows. Standbys and swings are other types of "covers" who are not regular cast members, and therefore do not perform during the run except in case of emergency.
A typical weekly performance schedule for Broadway is given here: http://www.playbill.com/article/weekly-schedule-of-current-broadway-shows-com-142774
In my theatre experience (not Broadway) the principal understudy plays a minor supporting role or in the chorus. If the understudy comes from a supporting role, then another chorus member is assigned to cover that role when the understudy takes the lead. The principal understudy does not only cover the lead in case of emergency, but can cover for any same-gender part in the play - they have every actor's part memorized.
More about understudies and covers here: http://www.newyork.com/articles/broadway/8-things-you-didnt-know-about-being-an-understudy-on-broadway-70460/
It's a gruelling profession. Broadway actors must be in shape, use good technique, pacing and restraint. Poor vocal technique will result in loss of quality, reputation and employment.
