The payment only needs to be above the minimum payment per hour to be fair. You could test that by first asking a few participants and see how long they take.
More importantly however, for the sake of your study, the payment also enough to attract all your participants back. Very low paying studies get participants because their is a large subject pool. When you want the same participants to come back it is going to be more difficult.
So, you might consider attempting to appeal to their sympathy writing to them to say that you are sorry that you forgot to add one question that is essential to your study, and why, and would you be so kind as to come back? kind of thing.
This may be especially effective since your first payment was so high. You may wish to mention that in some non-overbearing way. “I hope everyone enjoyed participating in our recent survey. We aimed to be generous with payments which averaged at $18 an hour. I am messaging you because…”
Bear in mind that some participants have turned off message notifications.
You could also start low, and then increase the payment to those who have yet to respond, possibly paying those who received the lower payment a bonus so that all receive the same amount.
I think that there will be a lot of variance when asking for “a few sentences,” and some participants may be inclined to interpret that as
"Often. Or as often as I want to. I like movies. "
As recommended recently you might consider breaking your question down into smaller ones to ensure you get more than the above kind of minimal response. E.g.
How often would you ideally like to watch movies? What other activities do you do at about the same frequency as watching movies? What constrains you from watching movies more? What is typically the trigger that gets you going to a movie?
If it were just a Lickert style response, more people would probably come back. Typing, on a phone, in a crowded bus etc can be fraught.
The longer you leave it the higher percent of respondents will come back.
Also it depends a lot on your demographic. Students may come back for a few cents but professionals may not be bothered to open prolific for less than a few dollars. That fact may encourage you to start low and increase payment offers.
I am very sorry I can’t give you a clear answer. It would be nice if Prolific would be so kind as to publish some data on this. You may wish to ask for that as a feature request. .