Eventually, you will pass out, as the oxygen concentration in your blood lowers. You'll get tired and drowsy. From what I've read, you might hear a ringing in your ears or see 'stars', so to speak (the blinking lights you see when you get up to quickly).
It can, depending on your dose, your metabolism, age and other factors, take time. 12 minutes according to the PPH, I think, but some users have said 20 minutes or even 30. I think I read here in a thread that, if you take SN in capsule form only, it will take an hour or so.