Any where from 10mins to a few days, when did you ad it? how old is the blog and is there content in place?
until google can work out what ads to display then they will display public service ads only (i think you can turn these off in preferances if you like)
yes right u should wait if u recently started. and sometimes service ads show up when no pin code entered after 6 month duration
As far as I know you can get displayed either alternate advertising or a block with the color of your choice instead of public service advertising. That is setup from inside Adsense's control panel.
If it doesn't resolve in a couple days I would throw up a page with some good quality content and see if you are still getting the psa's. I've seen some sites suffer from bad design and lack of good content for google to match ads to.