drinking too much?

I'm 10 days post op...is it okay that I can drink a 20 oz shake within an hour?

Yes.  As long as it doesn't cause you any discomfort, that is fine.  You cannot "hurt" anything by drinking too much or too quickly.  If you truly drink too much too quickly, the liquid can start to back up into your esophagus which is a very disconcerting feeling, but doesn't hurt and won't cause any damage.

Many people have no trouble drinking normally very soon after surgery.

Lora