2-3 weeks at #1, depends on the competition of songs out around that time. For example if a Blurred Lines, Royals or Happy comes along and smashes it out of position. The Top 10 has been pretty recycled lately though, there isn't much new material, and it'll start getting old soon. Plus a lot of #1's have been sitting there on average of 3-4 weeks (The Monster, Timber, Dark Horse) all of which, funnily enough, were pop star/rapped collaborations. Poor DWUW...I remember it winning all the "which is the best rap collaboration?" online face-offs because monsters are such a strong fan base.
But yeah, Birthday will come down to the competition that's around at that time. But listen to its radio-friendliness, look at Katy's track record, then come back to me and tell me it's not going to reach #1.
