I am still thinking at least 7 years. After 27(?) months, it is still difficult to get tickets and to date has sold-out consistently. That was definitely not the case by now with Kinky Boots of Beautiful (both of which had plenty of non-sellout weeks in their second year, filling 94% and 93.2% of their seats for the year; for that matter, Chicago only filled 97% of its seats in its second year.
I don't know if there is a backlash; I only know that 27 months of consistent sellouts is pretty impressive. I also suspect that, with its small cast and orchestra, it will be able to run for years at half its current gross and still make a profit.
The only reason I hedge at only 7 is because I do wonder whether its seriousness will work against it longer into its run, when a more significant portion of the revenue traditionally comes from tourists, who may not want to see it in the same way they want to see other, bigger or more fun shows (a la Lion King, Wicked, Phantom, Chicago and others).