It takes a regular koi thirty minutes, perhaps an hour to die on land. Merfolk being more advanced, can last longer- but too much raw air is indeed lethal.

[And despite having far too much fun being entirely doom and gloom, he will at least offer a tiny itsy bitsy shred of advice.]

Cup your hands over your gills. The longer those remain moist, the better off you will be. All else will be survivable. If you are able to acquire even a small amount of water, prioritize your gills.

And make sure anyone that seeks to move you comes prepared. Merfolk are generally heavier than the average human, and difficult to move on land.