From the EK FOM, Chap 4 Page 11 dated Aug 09 2003:
A crewmember rostered for standby duty shall, during the assigned period of standby, be ready to operate a flight at short notice should this be required. He shall:
.Be ready to depart from his residence or hotel to the airport within 30 minutes of receiving notification that he is required for a flight
And so on... In reality if I've needed more time crew scheduling have always made it available!