thirty seconds. the exact amount of time you need to open the shutter is depending on the light conditions you are in. if you are taking photos at dawn the light will change for every minute that pass so you can’t even use the same value for two photos to get the same result! simply start with a value of a couple of seconds to see if you need more or less. a too dark image need a longer time with an open shutter and a to bright image need a shorter shutter time or that you as a photographer wait until the sun has set some more.
use a remote 