Quick inspiration question

I'm trying to figure out if lifetap or inspiration is right for me. Lifetap seems straightforward.

Inspiration doesn't have the duration tag, so I'm not sure how to read the wiki entry in it.

If you are already on max charges, will spending Mana on skills still generate a charge but simply replace the others (basically keeping you at max) or do you need to reset the timer? If not, then Charge duration seems super important.

Do bonuses to duration increase the duration of the charges?

Since the gem itself doesn't show information about max charges or their duration, how do we know how it determines max number of charges and the duration?

Thanks in advance

"Gain an Inspiration Charge when you Spend Mana on Upfront" - it has no other conditions. It will act like a regular charge and refresh the cooldown when you gain one at max charges.

The normal quality benefits charge duration. If you can find generic charge duration (not like the ones on tree that only benefit power/frenzy/endurance charges), then this will also increase their duration. The gem doesn't have the duration tag but that doesn't matter.

The regular charges, and other charges too, aren't granted by a skill either, yet I imagine you know the default max charges for them, and their approximate duration. Slotting in Inspiration and testing for a bit will tell you what you need to know about the Inspiration Charges. Or you can look it up on the wiki.