You decrement the wish counter first, execute the action (which includes waiting those 5 days), and if it fails you increment the counter back. Something like this:

wishes = wishes - 1;
executeWish(wish).unwrap_or_else(|_| { wishes = wishes + 1; })?

This way if the action fails in the future, you get a wish back and can ask something else.

I always thought that wish-granting is instant, even if the effects of that wish are delayed.

So if I wish for something to happen in 5 days, it's granted in the moment and guaranteed to happen. That raises a question though: Can I wish to cancel a wish I have already made, but whose effect has not yet taken hold? On its face, this should be possible, but if we take it as a given that all valid wishes are always granted at the moment of utterance, then it might be physically/psychologically impossible for me or anyone else to revoke the wish before its IRL effect is complete.