dwayne
Adventurer
Assuming it is meant to basically give a free concentration I would consider it too powerful. The concentration mechanic is an important part of keeping casters from being too powerful and getting rid of the old days of having multiple buffs.
it is limited in the use as you can only have two rings on at once, also the speels effected are limited to low med and high and if you just hand them out well that's your issue for giving them away. as a player may only ever have one in his life let alone two. they are not candy to be dolded out. letting a caster for go one spell effect with con, is not like in the old days as they could stack endless amounts. if having more than one of these rings would be unbalancing then don't give more than one out.