Depends, 24-72 hours isn't uncommon. If you are using a bucket with a bad seal you might not see airlock activity at all. Airlock activity is not a sign of fermentation. It is just a result of it.
The only way to know you have fermentation is visible signs, Krausen, yeast swirling etc. Or by taking a hydrometer reading. If you pitched healthy yeast at a temperature that won't kill them into a sugar environment you have fermentation. Just be patient.