In the event of serious incapacitation of the elected president (illness, or severe mental degradation) the VP, or next in line, becomes indefinite acting head of the state. let’s say for 100+ days
I'm not sure what you mean by "de facto" here, since that seems to imply a certain degree of judgment. If a president is incapacitated, the presidential succession act makes the next relevant actor (the VP, the Speaker of the House, etc.) the actual president, as in, officially, name on the letterhead and everything.
Now, if say the 25th amendment gets invoked to remove e.g. a senile president, the succession kicks in. However, that might take a bit to get organized - is your "de facto" saying the timer starts from when the VP (or whoever) is doing the job, not from the moment they're official? Because that seems hard to adjudicate.