How specific does a Scrying target's knowledge of the casting have to be to decide to fail?

Question

After reading this question, I remembered an issue I've had with the Scrying spell for some time. Specifically, the spell states:

If a target knows you're casting this spell, it can fail the saving throw voluntarily if it wants to be observed.

How detailed does the target have to know? Is it enough to say "I'll scry on you sometime in the next year", or does it have to be as detailed as "on the 23rd Oct 1491 at 12:31:09, I will scry on you"?
If it isn't the latter, the target would either have to notice something when it's scryed upon, or alternatively decide to fail all scrying saves against scrying made by you.

How specific does the target's knowledge of your casting have to be, then?

Answer 1

It appears to be intentionally vague, so it is up to the DM to figure out the circumstances. For example, Alice gets kidnapped, and Bob wants to scry for her. She may not know that Bob will scry for her, but she might hope he does. Or, Charlie might have fallen and can't get up, so he chooses to allow all scrys in the hope that someone out there is looking for him.

The only real logical requirement is that (a) the target must know that scrying is a thing that magic can do (e.g. a year old infant can't automatically fail their throws), and (2) they're expecting a scrying, either because of circumstances or by arrangement. The DM might impose additional restrictions, but it seems reasonable that a target may consent to a scry for whatever reasonable circumstances might allow it.