You may only purchase or recharge one EXP ring (other EXP rings being the Empress Band and Emperor Band) once per conquest tally.
Using the final charge of an enchanted item will not cause its reuse time to be set to the maximum (16 hours, in the case of EXP rings). Because of this, it is possible to use the final charge of an EXP ring, recharge it, and be immediately able to use the ring again. (Be sure to use up the first Dedication first before applying the second; the two charges do not stack together.)