So far as I can tell, Prolific "guarantees" payment at a "minimum wage" rate. I say this because I know they assert this somewhere on the site, and I once emailed them after I tried to do a study that had a lower stated rate, and complained, and they emailed me back to say that they would require compensation for any underpaid studies. (As it turns out, I later actually did another version of the study I was complaining about, and there was some bonus involved, and it probably came out to the minimum stated rate.)
I don't know how this guarantee is supposed to work in practice. Most of the studies I've done since then have not excessively overrun their expected time, but I haven't calculated. I know they did send out emails trying to get people to do the study you're discussing, and I clicked on it, but threw it back since I can't do Inquisit since I'm using linux. I'd bet that since they're putting out extra effort to get this study done, they'd be willing to listen to any complaints.