When you first get it from Eluned, it has four charges. When you've used all the charges, talk to Eluned and she will recharge it for a fee. Each recharge at this point onwards will give you three charges.
The recharge fee is similar to recharging Crystal Weapons. It decreases after every recharge and it will stabalize.
If you encounter the teleportation random event while teleporting to Lletya, your charge will not be deducted and you get a free trip to Lletya with some runes.