Yep, I wouldn't feel warm and fuzzy at anything over ZERO. They can handle a bit, but long term exposure to NH3 is well documented as VERY unhealthy for elasmobranchs.
Check out the publication called Cellular and Molecular Approaches to Fish Ionic Regulation for starters. It's a bit older, but it has some good info if you can get your hands on a copy.
There was a published paper awhile back about how elasmo species react in high ammonia levels. They studied sharks, but the idea applies the same. It talked about the negative impact on the gills, and how that lead to issues with pulling the needed O2 out of the water. It also looked at long term effects of blood chemistry in relation to exposure to elevated NH3 levels. That information was actually used to compile a widely used standard for water quality during long term transporting of elasmo species, since the ammonia level is one of the primary killers during transport.
Also check out the sticky at the top of this forum called "Why FW Stingrays Die: A guide to prevention". It's got a lot of good info. Not all of it translated directly to marine rays, but the same negative impacts happen.
And welcome to the site, Padawan.