Roro wrote:

I thought the collocation [be+~ing for hours][till ~] would be acceptable.

Hello again Roro

It works as a restatement or clarification:

"It looks like I'll be studying for hours tonight – till 1am, at the very least."

"It looks like I'll be working all night – or until I fall asleep."

MrP

Thank you, MrP, too!
For your thorough clarification, which I needed (I'm sorry I was not clear enough about what I understood, what I got from your explanation).

Please let me think them over; .. I have just a little question now, may I ask you?　You say:
┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈
1. Up to 9pm, I will have been slaving away for hours.

The future perfect here has no peg to hang from (here, in this sentence as it stands).
┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈
Thus this sentence is not wrong when it's placed in a proper context, right? which give some 'peg to hang' in the future.
(If I'm wrong, please correct me, but if not, it's not necessary, I'd like to ask you a bit further when I grasp my question more concretely, if you don't mind.)

Then .. h'mm .. how about this?

[2] Up to 9pm, I will be slaving away for hours. (or: for one hour)

[3] Up to 9pm, I will be slaving away.

(My conjecture: in [2], again, there's some conflict between 'up to~' and 'be~ing for hours/for one hour' ..??　[3] is .. ok?　Of course what I want is your first impression and not some absolute rule.
Please don't put any stress on 'for hours'

It's morning here, I'm not wide awake, so please forgive me if I missed your points. I'll study these examples carefully, they give me great help. Concrete and easily perceptable.)

With my warmest regards,

Hello MrPedantic! Great Help! All the more grateful because it appears I had a wrong sense before.

I'm interested in those problems because there should (or could) be something in common in many languages.
(I have in mind here the difference between by/till, in particular.)

Thank you so much for all your help. See you,