Our approach has been to never add more than one thing per quarter. This gives us something small to adjust to, and gives us time to make sure the change is manageable. This goes for academic studies as well as for out-of-home extracurriculars or regular non-curricular at-home activities.
So for example this year my 11-year-old has added some regular work on animation, using the animation lab at our local school. It takes a sizeable chunk out of her week: 4 to 5 hours every Friday. She also wanted to take an on-line history course, but as per our rule about only adding one thing at a time, we're waiting until December on that to see how her week pans out with the addition of the animation before moving ahead with that. (She also joined the cross-country running team this fall, but that took the place of another sport that she dropped so that counts as a swap rather than an addition.)
To me two to four hours a day sounds like a lot for elementary-aged kids. My 11yo is doing some high school work (math and science) and her academic schooling still takes less than 2 hours a day. But combined with all her other passions and pursuits, that ends up being more than enough. While she does math and science most days, she rotates through various other subject areas from day to day and is still managing to make good progress in everything. My youngest, who is at a 4th/5th grade level on average, spends 30-90 minutes at academic schooling.
My eldest will be 17 soon, so we've been juggling and balancing for a lot of years. This "one new thing per term" approach has worked well for us in avoiding burn-out. If we take on too much it's only by a little bit, since our changes are always incremental. And we're willing to drop back if it seems too much, rather than assuming we have to get used to it.
So far the only person to really over-do it has been me, because I forget to play by my own rules and often take on too much paid work or too much volunteering, in addition to supporting each of my four kids' new things.