How long before the world explodes implodes stops dead in its tracks?
I’d give it 3 minutes. No power and no internet means no phones. People can’t live without their phone … not even for a minute. They go berserk if there’s a power outage, and if cell service goes down they don’t know what to do (How do I breathe? Help me, Seri!).
A nightmare scenario, but luckily these systems are extremely robust and the chances of it happening are extremely low even in just one country.
The greater worry though would be warfare, in which one country could severely damages its enemy's communications systems.