Grommets were stated as being approximately 24 inches apart. Approximately is right: the spacing varied all over the place from 14.5 to 27 inches ---AND--- they don't even line up from side to side so if you're thinking you can run a rope straight across from one side to the other, forget it. Absolutely crazy.
This was on a 12 ft x 20 ft Super Heavy Duty Silver-White poly tarp.