When you have at least daily 50-100 unique visitors on your hubs. You should have at least 10 featured hubs. Your hub account should be at least 1 month old.
These are my personal searches, but these are the facts most people don't know when they apply for Google Adsense.
You can apply for Google Adsense as many times as you want but to me you should follow these steps to let them not reject your first application.