How selfish, stupid, ungrateful, etc. can you get?
Definition 1 of 1
Pro Tip 1/3Expression of Disbelief
Used to express shock or disbelief at someone's extremely bad behavior. 
You missed your best friend's wedding for a concert? How selfish can you get?
Pro Tip 2/3Rhetorical Question
It's a rhetorical question, not expecting a real answer, but to emphasize the point. 
He always takes the largest piece of cake. How greedy can you get?
Pro Tip 3/3Stress and Intonation
Emphasis is often on 'how' and the adjective, reflecting disbelief or criticism. 
She never thanked you for your help? How ungrateful can you get?