I don't know if there is a particular time of day that Apple adds refurbs, but when I've been in the market for one, I check the refurb site several times a day.
You can also go to http://www.refurb.me and check out which models were previously available, and how long it's been since they were on the refurb store. They also will send you email alerts when a particular model is in stock, but I can never seem to get that to work correctly.
Made this thread last night, and Apple only had one low-mid range iMac in the refurb section. Randomly checked back a few minutes ago and they had the exact iMac I was looking for!
Yeah they all models of Refurbs in stock yesterday. It resulted in some more indecision for me for a while till I finally decided on a new 27" 2012 iMac instead of the refurb.