How much detail should sub plans have?

Enough that someone who has never met the class could run the hour without asking a question. In practice that means timings, where the materials physically are, and named people to ask - and much less rationale than teachers tend to write.

What should I leave for an emergency absence?

A prepared generic folder of two or three self-contained lessons per class, printed and in the room, reviewed once a term. Writing plans on the morning you are ill is the situation the folder exists to avoid.

Should I leave the actual lesson or a filler?

Leave real work, but the version that does not depend on things going well. Filler wastes a day and the class knows it; a lesson requiring a lively discussion with a stranger is a bad bet.

How do I know what actually happened?

Ask for a short written note, and collect the work in one named tray. An exit ticket does this on its own: you get back a stack that tells you who understood the lesson, whoever taught it.

What to leave for a substitute teacher

A substitute needs three things above everything else: a timed sequence of what happens when, materials that are already physically in the room, and the names of two reliable students and the teacher next door. Most sub plans fail not because they are too short but because they assume context the substitute does not have.

Write for someone who has never been in your room

You know that the class does the register while unpacking, that the visualiser takes a moment to warm up, and that the second group needs the timer on the board. None of that is anywhere except in your head, and its absence is what turns a well-planned lesson into a difficult hour.

The test to apply to every line: could someone who has never met this class do this without asking a question? If not, the line needs more detail or should be cut.

What to leave

A timed sequence: Not "work on fractions" but "9:05-9:20 warm-up on the board, 9:20-9:50 worksheet, 9:50-10:00 exit ticket, collect". A substitute is managing time in an unfamiliar room; give them the clock.
Materials, already printed: Physically in the room, in a labelled folder. A plan that requires the substitute to find the photocopier, a login, or your drive is a plan that will not happen.
Something more than the lesson: Work runs short far more often than it runs long. Leave a task-card set or a second activity that needs no setup, or the last ten minutes become the hardest part of the day.
Two named students and the teacher next door: Who can be asked where things are, and who to go to when something is genuinely wrong. Name them; "ask a sensible student" is not usable.
The non-negotiables: Medical needs, allergies, who leaves for intervention and when, who must not be seated together. Keep it factual and brief, and do not write anything you would not want read aloud.
Where the work goes: One named tray or folder. Otherwise it comes back to you in a pile with no way to tell which class it was.

What to leave out

Long rationale, your assessment plan, and anything requiring judgement about the curriculum. A substitute cannot act on the reason you sequenced a unit a particular way, and reading it costs them time they do not have.

Also leave out anything that only works if it goes well. A lesson depending on a discussion catching fire, or on students self-organising into groups, is a bad bet with someone the class has not met. Choose the version that works flatly.

Prepare it before you are ill

Emergency sub plans are written badly because they are written at 6am while ill, which is precisely when you cannot judge what is missing. The fix is a generic folder prepared while well: two or three lessons per class that are on-topic enough not to waste a day, but not so tied to sequence that they break if used out of order.

Review it once a term. A sub folder written in September against a unit you finished in November is worse than none, because it looks current.

This is also the one case where generating a plan is genuinely better than writing one at the time: a lesson plan with objectives, a materials list, timed activities and an assessment is exactly the structure a substitute needs, and it is the hardest thing to produce well when you are ill.

Questions

How much detail should sub plans have?: Enough that someone who has never met the class could run the hour without asking a question. In practice that means timings, where the materials physically are, and named people to ask - and much less rationale than teachers tend to write.
What should I leave for an emergency absence?: A prepared generic folder of two or three self-contained lessons per class, printed and in the room, reviewed once a term. Writing plans on the morning you are ill is the situation the folder exists to avoid.
Should I leave the actual lesson or a filler?: Leave real work, but the version that does not depend on things going well. Filler wastes a day and the class knows it; a lesson requiring a lively discussion with a stranger is a bad bet.
How do I know what actually happened?: Ask for a short written note, and collect the work in one named tray. An exit ticket does this on its own: you get back a stack that tells you who understood the lesson, whoever taught it.