I saw two schools of candle magick :
School one : letting the candle burning til the end, never snufffed out (only if you need to go out)
School two (Raymond Buckland) : let it burn til meditation. Snuffed them and repeat the all ritual for many days.
Which one make more sense to you?
Most people do not have the time to sit with a candle until it burns down, unless the candle they are using is very small, so School of Thought Two is the one that makes the most sense to me, though I have done the first option on occasion.
If the all point of candle magick is focusing… I also think letting burn while you do something must be counter productive 
I think it also depends on the type of spell being done as to which option is better. If the candle is dressed and charged, then it can generally be left to burn down as the energy is released by the flame. However, if the spell uses the visualization and intention of the magician, then generally, it can be snuffed or pinched out once your focus wanes ,and then relit at a later time.
So this is a very good question, I think it would all depend upon the specifics really for example let’s look at it like this shall we.
First School Of Thought - Let It Burn
So if you got a candle which is anointed, fumigated, with herbs and spell-powders mixed into it and dressed all over it. Each ingredient lets say is activated and the entire candle has been charged with the energies pertaining to the goal and/or consecrated too. One can place this candle and light it, the very moment the candle is lit and is fully awakened. The energy already begins leaking into the environment, and carries that magick into your life. Of course meditating, visualisation and energy work is usually done along side this. The major thing here is leaving the candle burn without conscious interaction with it, since the candle has been prepared in a unique fashion.
Simply leaving it do what it does, is leaving that gateway open for the powers, energies, forces and intentions contained therein. So if it’s done this way simply leaving it would be akin to spatial impregnation in a sense but into your sphere of influence and into your life or towards your goal/target.
Second School Of Thought - Repeat It
Now let’s say we take a different approach and you create something like a jar spell for example contained with a petition, sigils, herbs, oils, energetic links and various other implements. The candle magick part of this of course, is the candle melted to the top of the jar. Lighting that wick as the flame acts as a gateway, now extending your energy, the vision of your desire, your willpower, intention and so forth into the flame of the candle. Charging and consecrating the entirety of the spell consciously, investing more into it will likely result sometimes in greater potency and better results.
Extinguishing the flame on the wick once you’ve invested your desire, energy and willpower into it. Now let’s imagine at the moment that the next day you find yourself still desiring that desire, you return to the candle light it again, accumulating that desire within yourself pushing it into the candle in accordance with your energy, desire and willpower.
Conclusion
Which one works better you may ask, I think this would depend upon the specifics really, but from what I’ve seen both work pretty well. Sometimes one school of thought may work better with your chosen ritual format or sort of intention.
I’m working on a candle spell right now. I prayed over a pink candle (lit during the hour of saturn) and asked for a lot of potential boyfriends to come into my life. I plan on lighting the candle and relighting the candle if I need to. It all depends on the size of the candle and how much time you have to burn it down. I find that small spell candles can be burned down in one sitting, and ritual candles (as well as 7 day candles) can be lit multiple times.