Well that was embarrassing. We just ran a 100 participant study and had estimated it to take 20 minutes per person. We had set an upper limit cut-off time of 35 minutes thinking that would be ample (21 people took longer than this L). We were paying people 2 pounds for a 20 minute study (equiv of 6 pounds an hour). Turns out though that 71/100 of our participants took LONGER than 20 minutes to complete the study.
Luckily, Prolific Academic lets use give people Bonus Payments, in bulk. So, using Excel, we identified those taking longer than 20 minutes and just paid them for the extra time they took.
But, dear participants, apologies for this mistake. And, thanks Prolific! For making Experimenting that bit more ethical J.