The rules say rings... "can be recharged up to the maximum capacity of 21 charges by any Sorcerer of 8th Rank or higher. The time required for this is one lunar month per charge."
I think that's pretty clear. It takes 28 days to re-charge a ring. As with all Artifice-related activities, the work "is intensely demanding and ties up most of the Sorcerer's occult energy over a long period. No adventuring is possible while making one of these items." (By making, I would also include 'recharging'.)
These months
are inconvenient, but so is making many magical item. If it weren't inconvenient, the players would have no reason to be careful with the charges - simply blow everything like a machine gun and recharge at the end.
Ordinarily I'd agree with an option for a quest, but in this instance I'd be careful. Quests give plenty of rewards (in terms of XP and, usually, treasure), so this is a reward for squandering the item's charges...
And why would there be a 'quest' for something that is well understood by Sorcerers of high rank?
Of course, the exception would be where the party don't have an 8th Rank Sorcerer, so they have to find one and (no doubt) he won't recharge their precious ring "for free". He may have a
job for them...
(Are your players' characters below 8th Rank? Because, if so, they need to find the favour of a Rank 8+ Sorcerer
who isn't busy doing something else...)
This is one example where cultivating the friendship of a high-ranking Sorcerer is worth more than mere gold or jewels (as in the adventure 'The Greatest Prize').