It'll depend on how much charge the PB had when you got it. No more than two hours I think, usually.
Once it's done charging, go ahead and use it unplugged until you drain the battery, get the 'reserve power warning' and keep going until your screen goes black. The plug it in to charge it fully again. Rinse, lather, repeat once or twice more.
And congrats on the new purchase. Kinda frustrating to let the new toy just sit there charging not playing with it, isn't it?