There is no set time. Requestors post hits at all times of the day and night. You just have to be on the computer whenever a high paying hit gets posted.
It's hard to predict really! I try to check in numerous times during the day, since you can never tell when good HITS will get posted.
There's no fixed time, Requesters post HITs as per their convenience. You need to be at there at the right time to grab the good ones. Ok, let me reveal the secret to you. If you really want the good HITs and earn more at Mturk, you need to be present there at x o'clock, where x = variable. If possible, please BAR [BURN AFTER READING] because some things are better left top secret!!! Hope this helps!!!!!!
It sems to me that the best hits are posted in the AM west coast time, America. That's when I see all the article hits etc. I'm on mturk usually from 8 AM to 11 PM and that is what I have observed.