My wife had surgery for hiatal hernia repair and gall bladder removal 2 days ago. She has constant nausea- how long should this last?

Pain medications? If you are taking pain medications, they may cause nausea. However if this is not the case and the problem has been present for more than a week or getting worse at any time, do check with the surgeon asap.
It will probably ... Resolve in the next few days. 2 days postop is still very early postop -- the stomach and intestines may still be somewhat sluggish. Her surgeon should be able to prescribe some kind of anti-nausea medicine. If it continues to be a bothersome problem, follow-up with the surgeon.
If only 2 days post. Op, ; with no pain, just a couple of days more ambulate if you can- my spouse was actually laid up for over a week with post op symptoms.