When such feedbacking is done which comprises of NOT gates it is known as astable multivibrator and also acts as square wave generator ..
Now the time period of such square wave generator is given by :
2n * tpd where n is the number of NOT gates involved in feedback circuit and tpd is propogation delay of a NOT gate
So as only 5 NOT gates are involved in feedback circuit so n = 5 here..
Hence time period of given square wave generator = 2 * n * tpd
= 2 * 5 * 2
= 20 ns
Hence time period will be 20 ns..