Usually the best indication of a major show being a “success” is the point at which the producers of the show recoup their investment. After that, all money coming in is profit, as opposed to paying back the money that was put into the show.
On average, a major Broadway musical is going to cost 10 million dollars or more to produce, so it takes a hell of a lot of time to make that money back, hence the amount of failures (re: not recouping the investment) on Broadway.
If a show is selling at 100% or more capacity (standing room included) and the ticket sales were high enough, you could probably recoup in 12-18 months. So that’s 8 shows a week, makes 400-500 shows. If a show is steady but not skyrocketing it may take two years or more.