Anywhere from 0% to 100%, depending on vitality, age, species, timing (now sux cuz the leaves are forming), aftercare, site conditions,
et cetera.
20-33% is a common range people throw out but that means little imo.
Surprising answer, but i guess covers pollarding, seasons, storm reparation etc.? But as a rule of thumb of general, day to day..
i'd say in regular season, to look at the leaves/ this year's growth as about the only 'dynamic weight' that is feeding the rest of the enterprise (which as years go on is more and more static weight, that must be fed by the dynamic eventually the dynamic can't keep up, and a downward spiral starts). So, to me 20% is fair, 25% starting to push it, 30% pushing this edge hard. We can of curse take more, but i think that gets outside of the realm of a 'moderate maximum' of health on most occasions. Just because the adaptive wonder survives, doesn't mean that the range is maximum, just that the range is somewhere before failure.
To me, a tree is an orchestration of life; even the earth below it a soil sea. Many times i compair it to an aquarium. Lots of lessons there! "Harsh" water changes can harshly alter the formula balance, and once't again; 20-25% comes up to leave enough good behind to carry the whole and recover/recoup. So, this echos to me, across this broad range compairitson; than many forms of life (logically), are best left in that 75-80% remaining biology to carry on and re-populate to self equalize supply to need. This is to feed, stabilize, displace infectious activities etc.