Let's go out on a limb and say that the amount of "mental resource" - ability to perform mental work - make decisions, solve problem, generally complete tasks - in a given day is limited.
This wouldn't be precisely quantifiable - it depends on so many things - person, rest, physiology, nutrition, nature of activity, and possibly many more factors - and we aren't after quantifying it precisely.
Sill, you'll probably have a pretty good idea roughly how much of what work you can handle before your eyes glaze over, you start spacing out, slowing down and eventually just freeze and even doze off..
The emphasis is on this being depletable ; Simply put, one gets tired sooner or later.
Why discuss something so obvious and not even precisely quantified ?
Because I found it to be particularly helpful to be mindful of, to figure out what this is spent on, what drains it, etc.
Particularly relevant are the related hypotheses that certain things "burn" through this at much higher rates than anything else
* Conflict / stress is extremely costly, leading to a rapid and lasting draining of this resource, productivity, memory etc. plummeting, things getting forgotten - with some people being more sensitive than others to emotional engagement - Keeper personalities perhaps more than average - I know this is true of myself.
* Simply being in the presence of an IHSSS is costly in a background way - because one is constantly on alert ( in a state of activation ), expecting one shoe or another to fall, even without anything specific happening.
* Child care is extremely costly as well - child also being an IHSSS of sorts, obviously without malice, fault or awareness, and with the understanding of the sacred duty of the parents - constant stream of requests for help, attention - but it's the high-stakes, intermittent nature of it that does a number on the FMR.
* Intense, critical, multi-person job engagements - such as IT person getting engaged in an incident response.
* Decluttering / Organizing one's possessions - this may involve lots of decisions big and small, moving around, finding or creating places for things, deep introspection, etc. - times a lot.
* Multitasking compounds the drain - having to juggle multiple things in one's head on top of everything - even as each separate task may get decreased performance.
* etc.
Even though not discussing it in any quantitative ways, I still find it immensely helpful as a concept.
The key point being awareness of the impact of these different factors that deplete it faster, and some of them being avoidable, or at least known.
Has anyone noticed how, arriving home after a hard, busy, intense day - or even after a large social gathering - one just crashes on the coach and rests, or even falls asleep ?
Or, how after a significant conflict, one is unable to focus for quite some time afterwards, and feels depleted ?
This is the point.
For example, I recall a specific occasion, where I had to print or copy something, but there was this conflict with the spouse happening - over text messages, even - but very intense - and I just kept going in circles around the printer, starting, getting interrupted, forgetting, answering, starting again = for no less than half an hour, significantly more than the simple task warranted.
This is why Nagging and Yelling, or any conflict, or even just low-level negative critical remarks in passing, micro-bursts of displeasure, just overhearing emotional engagements between other people, etc. can have such disastrous impact on absolutely every kind of work that needs to be done. The alarming event hijacks attention and mobilizes all resources in a "fight or flight", lizard brain level response, also burning through the FMR in the process, as the mind gets "overclocked".
Spend it wisely, watch where it might be getting overspent, and manage or avoid what you can.
Know yourself and what you are up against.