There is no best survey-in time, as its duration will mainly depend on what accuracy you want to reach. This setting can be set up with either Mission Planner or QGroundControl, as well as the UCenter software in case you don't use an autopilot. Keep in mind that having an efficient survey-in will require to have the base set in the most building-free space and a clear sky-view. The time necessary to reach a 1 meter accuracy for the base positioning is approximately 10 minutes. Reaching 50 cm and below accuracy will require a much longer period of time, roughly an hour, or more.