Not so far

Akko narrowed her eyes as she slowly pieced together what had just happened. She had been at the table, then a few feet ahead, and then back at the table. Which means... "The ring can teleport me!"

And she quickly covered her mouth, remembering that she was still in a library. She waited a few moments and, seeing that no one had scolded her, Akko turned her attention back to the ring, wondering where the teleportation limit was.

Walking towards a window, Akko saw one of the gardens surrounding Luna Nova, somewhat far away. Wanting to test the ring, she tried to concentrate like she had before. And, in another puff of smoke, Akko had disappeared from the library... leaving a pair of pink panties lying on the floor.