Great question, @Berta!
If you have to leave the area, it can be dangerous to let candles continue to burn unattendeded- the most important thing is staying safe! Of course, this leads to plenty of half-finished candles. @Walter summed it up very well- candles for general purposes, your altar, decoration, etc. can most definitely be re-lit and re-used without worry 
Candles used for spells meant to absorb or draw negative energy should likely not be reused, as they take on the negative things you were removing/drawing away from you. For candles used for these purposes, consider disposing of them in a safe way if you can’t let them burn all the way out.
For candles used for other types of benevolent spellwork, I personally believe it comes down to your intuition about whether to re-use them or not. @Praecog29 makes a great point about your intention- how you feel about reusing/disposing of your materials really is the most important part! Go with your instinct and what feels right to you.
Great question and good luck, Berta! 