I dunno about that. Before she got sick, she did 120-140 royal engagements a year. Now, some of them would have been on the same day, but say she did about 90 days of engagements in the full year. Now, given most of the engagements are for charities, or hospital wings, or museums, she has to actually know what she's talking about when she makes a speech or meets the staff. I would say for each engagement she'd have to do at least 3/4 hours of being briefed on the history of the charity, the art in the museum, who the main management are, and then hair and make up of course.
This stuff doesn't just tick over, everyone who has ever met a royal on these occasions are always very effusive about how much they know. This sh*t is a job. A stupid job? Yes. But a job all the same.
If you add on the prep time, you're probably saying two days a week of engagements, with a 1.5-2 days of prep time. Not exactly a taxing schedule when compared to the rest of us but also the rest of us don't have a bank of reporters following us on our commute and then sitting down beside us in the office reporting on what we're wearing, were we nice to everyone, did we look engaged at that team meeting, did we step out of line at any point. It must be utterly exhausting. You honestly couldn't do it for more than a couple of days a week.
And I sincerely doubt ANY of the engagements take 15 minutes. No way does Princess Kate show up to the opening of a museum wing, or anything, and get in and out in 15 minutes. It'd take 15 minutes to just get through the front door with all the handshaking and little-girl-flower-taking.