[mirimir]

I'm not sure what "butt-in-seat" means, unless you're working interactively in real time with a team, or doing customer support. Otherwise, what matters is billable time. That is, time when you're actively on-task, and not on HN or whatever. A reasonable practice is tracking 10-15 minute intervals.

Then limit that to a rolling average of 40 hours per week, or whatever you like, and can negotiate with your employer.

I've mostly worked as a consultant, so there's been no limit, other than however close to 24 hours per day that I can handle. And sometimes, with rush jobs from multiple clients, I've just about maxed. But in that case, I've discounted work time, especially for clients who weren't pushing as hard.