On the limit
In F1/motorsport, you've probably heard the cliché that the driver was "on the limit". Essentially, it means they couldn't go any faster. Try to push the car harder around a turn, and you'll lose traction, spin out, then hit a barrier.
I've found this idea of being "on the limit" to be a useful analogy for things in my everyday. Whether it be the number of focused hours I'm able to put into my work, the reps in my gym routine, the experiments on my sleep, or any of my other hobbies. For me, finding that limit is a similar feeling to ripping a car around a corner. You have to feel for the grip available in the tires and listen for any screeching. Once you've felt the car slip just the slightest bit that first time, you get a good sense for the car's maximum capability. Same is true in life. To really find what you're capable of, you just need to push like hell until you slip. However, slip too far or too long and you'll just end up with diminishing returns, burnout, injury, or frustration.
Find the limit. Stay on the limit. Adjust, as your limits inevitably change.