Your comment made me realize I didn't know. So I looked this up, and it's actually a bit of both. It depends on if Congress is in session when the 10 days have passed:
If any Bill shall not be returned by the President within ten days (Sundays excepted) after it shall have been presented to him, the same shall be a Law, in like manner as if he had signed it, unless the Congress by their Adjournment prevent its return, in which case it shall not be a Law.
And it's disgusting