Hi
It is 30 minutes that is usually recommended, but if you reboot or turn-off the router the other end should get to know that and not think it is a noise problem that it needs to slow your down your connection to try and resolve.
I think the worse thing you can do is just pull the plug out the socket, rebooting or turning off the power to the router/modem is not quite as drastic.
For a one off turn-off or reboot I've never waited and it's made no difference to my line.
Regards
Phil