Per-encounter-ish. If you cast hex, which is a biiiiiiig chunk of a warlock's damage, then you will only have 1 spell slot afterwords to spend on any of the warlock's spells until the later mid and late game... which takes a loooong time to get to in practice, and by then you will have better spells for those slots than DW. Seeing as there are typically 2-3 encounters/ short rest, using DW will only really give you a benefit one round of one encounter, which is likely going to be a bit less useful than most of your other options (which are either potentially long-term buffs, or potential encounter-winners like Hunger of Hadar, Fireball or Evard's Black Tentacles).