Great question! & it has come up in the past.
Of course first & foremost is always Candle Safety.
You can also dispose of the candle safely
Personally, I do have a specific candle on 1 of my altars for Brigid that I relight whenever I am working with her or would like to invite purification into my space. I have another one on my other altar specifically for protection that I will snuff out & relight with the same intention when I relight it.
If the candles are anointed or carved into for the specific spell & it’s say… a tealight or chime candle, I will let them burn to completion.
For other ideas & interpretations on how to use candles, dispose of them, or reuse them… you can check out: