How do I skip the 90s timeout in systemd

You have two options:

You can set TimeoutStopSpec= on a specific UNIT to a specific value (in seconds*) to wait. You can also set it to infinity in which case SIGKILL will never be sent (not recommended as you may end up with runaway services that are hard to debug).
Set DefaultTimeoutStopSec= inside /etc/systemd/system.conf (or user.conf, or in one of the *.d directories) to a default value that all UNITs that do not have TimeoutStopSpec= specified will use. The deafult for this setting is the 90s you normally see.

Man page references:

_{* systemd also accepts time specs, e.g. "2min 3s". That's extensively described in the man.}

It is not possible. Systemd is noninteractive.

You can uncomment in /etc/systemd/system.conf the lines:

DefaultTimeoutStartSec=90s
DefaultTimeoutStopSec=90s

And change the value to what you consider appropriate.

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