Cut the candle in half before charging it. I use emergency candles (short white candles) for shorter spells. Magick isn't accomplished without dedication. I have done a 6 hour ritual before.
If you cannot finish for whatever reason, snuff out the candle, do not put anything away, do not break the circle, and come back as many times as is necessary. Cut the circle, walk out, and seal it back behind you. However some people do not even cast circles, it is a personal choice.
Some spells take 3, 7, or 9 days to complete and are done in this way.
I let protection and vigil candles remain lit and walk away, but that's because I've charged the candle to perform until it is burned out.
But when you have a spell goal, you should stay until it is finished while continually concentrating. The candle is then a point of focus, not the power of the spell.