The answer probably differs a bit depending on what counts as preparation. I rarely work specifically with one person. If someone wants that I do spend some time thinking about what they need and want and how to address that…. I would guess I am in the ballpark of the others in the 15 minute range.
With yoga I have taught it so long that I do not plan classes out move by move. I think in yoga it would not make sense anyway, as I will follow the flow of who is there and what I see in their bodies. I don’t take out materials and search out new poses for a specific class, because it would seem artificial to go ‘here is this cool new thing’, rather than using the cool thing whenever the need arises.
I do spend quite a lot of time taking other yogis classes, reading, practicing the more meditative and focusing techniques, doing postures, trying out sequences….. But do you count the general time spent enhancing one’s understanding of one’s discipline as preparation? Then it becomes a lot longer.