Time to replenish?

Count Choculitis

I'm confused.  I know that supply will meet demand, yadda yadda yadda.  But how long does it actually take once I'm essentially empty (after pumping for example) for more milk to come in?  Reason I ask is because I've been nursing my twin boys and I was told by the pediatrician to pump after each session.  Problem is, though, that oftentimes my boys are hungry again just 30 minutes after I nurse them.  But usually by then I've already pumped and I'm afraid that they're just sucking on an empty breast.  But as everyone knows, it's hard to tell what they're getting.  Anyone know how long it takes for your boobs to "refill"?