You can sleep on it right away. But it usually takes 4 to 6 hours for your mattress to reach its full plumpy gloriousness. Or if you can hang on a bit, we’d recommend giving it 72 hours.
Another hot tip: leaving a door or window open when you unpack your eve can help that new mattress smell to waft away. And don’t be alarmed if you do smell something. That’s simply the foam’s ‘box fresh’ smell and should disappear within the first couple of weeks.