I think to remember that with a former version of BOINC, I was able to set the snooze time period (at that time, only for the CPU).
Now I would like to set a certain snooze time period for the GPU, but I cannot find this anywhere in the Options (neither for CPU nor for GPU).
Am I blind, or can such a setting no longer be made by the user?

Going blind I would not know but a quick forum search reveals you can set gpu snoozing using boinccmd tool per https://boinc.berkeley.edu/dev/forum_thread.php?id=6071

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Coelum Non Animum Mutant, Qui Trans Mare Currunt

I don't think it's ever been possible to set a 'snooze' time other than 1 hour using standard Manager tools.

There used to be a 'Network reminder interval' - slider from 0 to 120 minutes - which survived from the age of dial-up modems until at least v6.10. Something like that could have been adapted to make a default snooze time control, but wasn't.

Going blind I would not know but a quick forum search reveals you can set gpu snoozing using boinccmd tool per https://boinc.berkeley.edu/dev/forum_thread.php?id=6071

referring to this part of the posting from your link:

You can set it using a GUI RPC command, e.g. on my Windows machine:
"C:\Program Files\BOINC\boinccmd.exe" --set_run_mode never 15

I have to do what exactly?

Here's the complete wiki info

--set_run_mode {always | auto | never} [ duration ]
Set run mode.
always: do CPU work always
auto: do work only when allowed by preferences
never: don't do work
If duration is zero or absent, this mode is permanent. Otherwise, after 'duration' seconds elapse, revert to last permanent mode.
--set_gpu_mode {always | auto | never} [ duration ]
Set GPU mode. Like set_run_mode but applies to GPU computation.

Seems to me if for instance you have computing set to auto and then wish to issue a snooze of 30 minutes it becomes

boinccmd --set_gpu_mode never 1800

Add the appropriate path front of boinccmd for execution. After 30 minutes the auto schedule resumes. Looks from last post by Fire$torm s/he had it going.

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Coelum Non Animum Mutant, Qui Trans Mare Currunt