https://www.qubes-os.org/doc/how-to-copy-and-paste-text/#automatic-clipboard-wiping
To enable automatic wiping of the clipboard after a minute use qvm-service:
qvm-service --enable VMNAME gui-agent-clipboard-wipe
You could also manually add it in the service tab of the qube setting.
The “1 minute” is not (yet?) customizable.
https://github.com/QubesOS/qubes-gui-agent-linux/blob/main/gui-agent/vmside.c#L57
If you want to change it, I guess you have to compile the gui-agent. 