>>13619184you dont.
I (mostly) only work on math I find interesting, and I take plenty of breaks. The "vigorously study for 16 hours at a desk doing mountains of exercises every day" is a meme.
For math you pick a theorem or example you want to learn. Then you follow it logically, filling in any gaps. Then you recreate it yourself. Then you explore the consequences a bit to get a feel for the implications. By then it will be in long term memory. This might take an hour, it might take a month. But as long as you find it interesting then it isnt tedious nor boring.