Fanning powder? Any time is a really good time, you'll definitely notice differences when you apply powder, don't think this is an instance restore to a brand new deck, cause it's not. Fanning powder makes the sliding of card against card a little rougher but smoothly so the deck acts as unit when you're doing fans. Try to do a one-handed fan with the whole deck without powder and then try with a powdered deck, you'll see the ease. Essentially, there is no right or wrong time to apply. People even apply powder when it's brand new, cause the deck is too slippery and makes exploding pressure fans. But even then it takes a little time to work powder into a brand new deck.
Powdering completely dead cards that have been through the wringer and back and few trips to the washing machine, don't even bother.