How to Make Your Book Permafree

I know a lot of authors on Kobo also use free first and series or free promos. I'm curious, do you have any opinion on whether it's worth keeping a book permafree or whether it's more valuable to set it free for a limited time so that it is kind of special for a short time? You know, like I said earlier, I think a lot of these things are specific to the author and the working techniques they're using. So I would encourage authors to experiment and see if they can get better results with either one.