I apologise for my strange question :-) I suppose that when I turn on an wifi device it emit continuously radio wave. What happens when I turn off the device? The last wave emitted how long bounce in the room? Thanks
In a small room, say 3 m wide, with radio travelling very close to 300,000,000 m/s, it takes about 10 ns to cross the room. Each time it bounces off the wall it will be very much attenuated. Perhaps it takes five bounces before it's entirely insignificant, so that's about 50 ns. It's pretty brief.