Probably within the first 12 months (f. e. when the new Ratchet or GOW games are released).
Maybe earlier if some PS4 games I haven't finished yet show good visual advantages on PS5.
But I think it is time to sell two of my three PS4 controllers now. Also my retail copies of Persona 5, God of War, Uncharted 4 and infamous Second Son (due to the PS+ collection).
If I can get $100 - $120 for the 2 controllers + 4 games it would be a nice incentive to make the PS5 jump earlier than needed.