How long does this last? For the rest of the time that card is un play or just for one turn?

It lasts until it is removed by another card's effect, or until the card it is attached to dies or is discarded or returned to hand.

Here's a scenario that a friend and I encountered with Imprisoned:

I had Imprisoned attached to one of my unique characters, then chose to kill that character to resolve a military challenge. I then played Not Today to save that character. My friend and I could not agree if the attachment left play despite the character being saved, or stayed in play because the character stayed in play as well.

Any thoughts?

Attachment stays because character never left play. Saving a character from dying doesn't make them die and then come back from the dead, it means they never die in the first place.

On the condition of imprisoned it says to kneel the character and they cannot stand during the standing phase. The argument we are having is if the card 'hand of the king' is used on the imprisoned character, will he be allowed to stand. Will that remove the imprisonment attachment.

The counter argument is that the only card that will remove the attachment is 'spoils of war' which allows you to remove an attachment.

Is there any other way to have the imprisoned attachment removed outside of the spoils of war card, and death?

Hand of the King

Attach to a Lord or Lady character only.

Any Phase:Kneel Hand of the King to stand attached character.

Cards only do what they say they do. This will not remove Imprisoned.

Character can stand in any phase (except the standing phase - because of Imprisoned).