After much deliberation and going back and forth from an hourly rate to a daily rate to a monthly retainer, I've streamlined my rates. I charge £450 a day as a flat rate.
If someone wants to buy half a day they can, but I normally suggest a minimum of 2 days per month (working out to half a day a week).
With half a day a week, it gives me enough time to deep dive into analysing the site's data and to implement actions from the analysis.
I aim to work on no more than 2 clients a day.