You can initially have up to three charges of each kind (power, frenzy, and endurance), which will last for 10 seconds - but as you've noticed, you can refresh this duration indefinitely.
This can be increased by an additional 3 charges and roughly 3.5-4 seconds (I'm not sure how those bonuses stack, but the difference is small enough) via passive skills.
Helping Alira in the Merciless Act 2 bandit quest provides another +1 max power charges.
Two-handed weapons can be corrupted to provide +1 max power charges.
The unique staff Hegemony's Era can provide +1 maximum power charges, or +2 if corrupted with the right mod.
The unique wands Void Battery and Storm Prison also provide + 1 maximum power charges, allowing for 2 more max charges if duel wielded.
The unique amulet The Aylardex can provide +1 maximum power charges.
This leaves us with up to 10 max power charges, depending on gear chosen.