(I don't know how long he will be in the show.)
Probably for the run of the show. I don't know if they have alternating casts so that the performers can rest for a day, but Wayne will do the show until he doesn't--whether the show comes to a conclusion or someone else is cast in the lead.
Actually, it's quite fair to wonder how long of a run Wayne signed up for. This sort of stunt casting, especially for a show that's been around a few years, is increasingly common, and rarely open-ended. He almost certainly signed up for a finite amount of time (usually in three-month increments) after which he'll wave goodbye and they'll continue with a no-name in the role or find some other vaguely famous person to do it for a while.
Chicago has thrived on that formula for years, with notables popping in for a while to goose ticket sales. Notably in our little world, John O'Hurley has played in
Chicago both on Broadway and on tour between TV hosting gigs.
No, they don't have "alternate casts". Wayne will have an understudy, of course, but they'll try to have him do as many shows as possible since that's who people will be paying to see. And of course this doesn't affect LMAD at all. He's not starting until November, so chances are he'll have finished the entire season of
Deal by then anyway.
