It will all depend on what they were fed prior, temp of water.. sometimes the little things make a difference like using IAL and dim lighting. Can also place a fan blowing onto the tank (as long as the tank isn't completely sealed off) so the warm water temp rising and the cool air above creates a pressure similar to stormy weather.
OFL is correct, the more natural the food the better most spawns will be.
It can take anywhere from a few hours to a week+ for a spawn to happen. I've had them all spawn within different amounts of time - some as little as a couple hours but then would have pair spawn a week later. Normally though they will spawn within 2 days if they are prepared and the female is eggy.
Don't bother them too much - watch from afar.. as long as they were conditioned properly they don't need food during this time. You can feed them if you want (I sometimes do), but keep interactions as little as possible as some are pretty skittish and it can disrupt them, make them uncomfortable and feel the eggs won't be safe. In time once you get some spawns going you'll be able to figure out what each pair does best with and their tolerance level and it will become a bit easier for you :) But even still you'll always have that one pair that just won't play nice lol