I don’t do much flatwork on days where I school over fences. I spend the days prior to that getting the flatwork I know I need to address tuned up. But, with that being said, I don’t jump every week, and I don’t tend to school courses much at all, so take that with a grain of salt. A big one. Like one of those chunks of Himalayan pink salt or something.
When I’m schooling over fences, I do about 15 minutes of basic flatwork (i.e. walk, trot, canter, work on adjustability a bit, done) before starting to jump. Now, that’s mostly because my mare is very particular about when she wants to do flatwork and when she doesn’t, but many of Kip’s lessons were like that - when the goal was to work on courses that would be at a show.
I would not expect a jumping lesson to go over an hour - if anything, I’d expect less than that. But to me, a jumping lesson where you’re working on courses is similar to a horse show; you’re seeing how well you can apply the flatwork that you’ve been working on. I wouldn’t drill the flatwork immediately before that because I’d want to see where I was, progress-wise, but I’m perfectly content to say that I’m in the minority there.