I did 200 earlier before most people seemed to be aware that they were there. Then I did another 200 something in the evening, and thought I would go mad so I took a really long break. Got dinner, played around on the web, thought I should get some sleep, went to check my stats, mTurk did not work - kept timing out for a few minutes, came here to see if it was just me, saw someone mention the 500 cap, realized I only had 60 to go, thought, "Sigh...just do them", did them, laughed like a moron on the last one, retake 4 times, got the message that I was capped. Tried again. Got another message that I was capped. Tried on a different browser just to see....got my third message that yes, damnit, you're f-en capped!
TL;DR - On and off throughout the day, but about I guess 2 or so hours total?