With candles it all depends on the individual candle, some candles will burn slower than others and others much faster. In an ideal world I would suggest having an identical tester candle burning to gauge the general time a candle takes to burn, unfortunately we aren't always able to do this.
In some traditions slow burns mean that the results of the working will take longer to manifest or will have a difficult time manifesting, while a fast burn can mean that the results will manifest quickly and easily, with little difficulty.
However, some candles simply do burn slower than others and the humidity and airflow of the room can make a big difference, as can the age of the candle, the type of wax, colouring and scents (if there are any).
I would say that if a large candle burns for an hour or so longer than expected that is pretty normal and just a variation in burn time, but if a candle meant to burn for 12 hours, burns for 20+ then there is something potentially more unusual going on.
In terms of whether someone else should light the candle for you, I personally wouldn't do that in my own practice. Mainly because it is your intention that is important and having someone else light the candle may interfere with intention and may slow down or even stop the results from manifesting, but it all depends on what your style of working is and where your beliefs on this are.
| 
