I've heard "lunchtime" used to indicate a time of day.
The difference between that and noon, for me, would be that noon is 12:00, pretty exactly, and "lunchtime" could easily be a half hour or even an hour later.

Yes.

If you said that you had to finish something before lunchtime I would think that you wanted to finish it before you take a break for lunch.

"Noon" would just mean there was a deadline at 12 o'clock (which might or might not coincide with a break)

(German native working in fairly international company)

*edit* *but too slow!*
PS: More common, I think, for your example sentence would be "I have to finish this before lunch" (rather than lunchtime) - this could mean either before lunchtime, or before I am permitted (or permit myself) to stop working and take a lunch break.

Thank you!

If it's a deadline, you would usually use by rather than before.

I have to finish this by lunchtime.

Agree with all. I do think there's a difference too. For example, "I read that at lunch(time)" means you read it while you were on break, which could be anywhere between, oh, 11:30 and 2-ish. So, not necessarily noon.

I think the helpful information conveyed in "I have to finish this before lunchtime" is that the person is saying they have to finish it before they get up and leave for an extended period of time. Meaning, if someone comes by and the worker is out to lunch, that means the task is supposedly complete.

Here's my impression of some of the options:

before/by noon = 12:00
before/by lunchtime = the time I or most people usually eat lunch
before/by lunch = a fixed time when lunch is served
before/by the middle of the day = the part of the day between morning and afternoon

before I stop for / break for lunch = while I'm still thinking about it, without interruption
before I take lunch = before I officially go off duty at lunchtime
before I go to / leave for lunch = before I leave my desk


To me 'before' is a little earlier than 'by,' which could include the stated time.