You can't run out! The reservoirs can be more or less empty, but you can't run out!
I certainly find that if DS2 has a greedy day, I have scary Jordan boobs soon afterwards, as my body just makes more and more milk.
Seriously, the reservoirs are more empty or full at different times (more full in the mornings, generally, for me), but they're never actually really empty - more's going in, all the time.