Question: how much time do you have left on snapchat when the hourglass shows up?
Answer: When the hourglass emoji appears, it means that you haven't been exchanging snaps with your friend within the past 24 hours. I've tested this, putting my snap streaks on the line, with different friends and they all seem to come at the 20th hour of no contact. Leaving 4 hours left until the 24 hours time is up.