It really varries. I think the beginning of the week, on Monday or Sunday is most common for new offers. About three weeks ago, they added a bunch of non-credit-card offers, like 15 pts worth. Last week they added a bunch of credit-card offers. Normally they add stuff in bunches, because the offers don't come from the websites themselves where you do the offer, but through a third party marketing firm.
The do sometimes add one or two offers, but usually it comes in groups. I would guess because they did a bunch of CC offers recently, they will try to add some non-CC offers in the next 2-3 weeks.
My referral link if anyone has yet to sign up, and wants to help me out.