Well you dont have to wait THAT long but if you can, more power to you.
6 months from your last approval (not a day sooner) should be good enough
With 3 approvals in January, gardening for the next few months is your best course though; good luck! 
Another thing too, Barclays says they like to see AT LEAST 1 year of credit history. I'm thinking, with that, and some of the approvals that people are getting at the one year mark, I'm going to make that a goal. Then Discover, Amex, Chase, and Barclaycard here I come! Well Barclay first actually, you know how they are lol