Celeste should be dropping Spellbound Dew as soon as she's won, but it won't be a guaranteed drop unless you've reached Lv. 5 buildings, as you shouldn't need Spellbound Dew until you're upgrading your buildings to Lv. 6. It won't be a guaranteed drop until then, but once you need Spellbound Dew for Lv. 6 buildings it will be guaranteed to drop between 2-4 Spellbound Dew each collection.