Depends what the goal is, and what your responsibilities are.

If you want to learn, the upper limit is literally just defined by remaining health. I'm pretty sure seniors can audit for free at least some places.

If the goal is to make more money, it depends what you're already making, your chance of successfully graduating, and how much you'd make in your new job. A simple equation that's close enough, but not totally correct would be something like: (new salary - old salary)*working years*probability of success - (tuition + old salary)*expected time in school

If it's to have a job you like more, it's hard to quantify and will be a pure judgement call.

Given that you're still working age it's realistically going to be a mix of all three. I'm also not sure how having kids would factor in, exactly.