It depends on the model of the smart glasses you have purchased.
ODG : Turn ON in 50 seconds-1 minute.
Google Glass : Turn ON in approximately 50 seconds-1 minute as well.
Vuzix : they need appromiately 10 seconds to be operational.
Realwear : Usually it takes 20 seconds.
Navigator 500: Usually it takes about 20 seconds.
Moziware MZ1000: Usually it takes 15 seconds.