I can literally give 1000%. But that is very much dependent on how I set my levels. By giving 100% on a daily basis, I mean taking what I can do over a decade and dividing by 3653. So when I give 1000% one day, that means I may only be able to give 50% for the next 18 days.
There are a couple of points about this perspective. I choose a decade because that is long enough to even out things like broken legs or pneumonia without changing too much because of aging. (This won't be true for the first or last decade of life, of course.) More importantly, if you define 100% as the maximum you can do in a day, then you will either be constantly falling short or you will eventually wear yourself out. Most critically, if you live that close to the edge, if anything happens, your life will fall apart. With my definition, you have some reserves for emergencies that demand it and are mentally prepared to recuperate afterward.