As far I am aware of, this study could progress forever till the end of pandemic and then conclude that it was effective all along (or perhaps not). They have not published any stopping criteria for the trial so far.
Well, they have published number of studies already. (principletrial.org/results) Some with some effect. I don't see why Ivermactin would be exception.
For some reason they have dragged this study for quite long time without any interim results.