I think it depends on the story you are writing.
Personally my fantasy is usually of the non-magical or very limited magical kind, (Yes, I know, and I write wizards, go figure) so I do not personally put too much effort in a magic system. Reason: It is not important to my story and my story can move forward without any magic involvement at all.
When I do use magic, I use it on a very limited basis.
For example: in my Twighlight Manor series there are a few *magical* characters: sirens, born with the ability to control electrical currents and water; phookas, born with the ability to shape-shift and *throw* fire; and an alchemist who spent most of his life studying how to control the energy around us and use it in a typical wizard like manner. The sirens and phookas are born with their abilities, and thus technically are not *using magic*. The alchemist is the only one who really does any magic that could actually be called magic.
As you can see, for the purpose of that series, I have no need to create a system, other than to write up a few rules about what these characters can and cannot do. The whole system easily fits in one paragraph.
The series constantly hints to an older period when wizards roamed the world and magic was a thing, but it also tells of magic going bad, being used for evil, and the people rising up and destroying all spell books, killing the wizards, and completely erasing from history the names of the wizards in an attempt to force the world to forget magic ever existed. This leaves the series open for magic to come into play, should anyone ever take it upon themselves to seek to bring it back, at which point I will need to do something about writing up a more detailed magic system.
But, as I said it depends on your story. If your fantasy is set in a place where magic is a daily thing and is done by many beings, than you are going to want to write up a system of some sort. Why? Well, that's easy.
If you have your ice wizard saying he can only perform a snow making spell if he has the tooth of a rare snow dragon, in chapter one, than you better not have him cast a snow storm with a simple wave of the wand in chapter 10!
The purpose of writing a magic system is so that you know who can do what, when they can do it, and what they need to do it with, so that you keep your story consistent once you start writing.
So my answer to your question is this: match your level of magic system creation to the level of magic used in your story. If your story has no magic, than no system is needed. If it has minimal magic, you would only need a few lines typed up. If magic runs rampant in your story, than you may very well find yourself needing a details set of charts, graphs, and a full 50 page glossary of which spells do what.
There is no right or wrong answer here, because it all depends on your story and your prefered writing methods. You can build as much or as little, in regards to the rules your magic needs to follow.
The list below are the original pages written in 2003, and republished on Squidoo in 2007:
Even more articles have been written for this series since then:
As there are now more then 100 articles for this series, it now has it's own index page on which to list them all, as there are just too many to keep adding them to this end of article list. You can find the complete listing of all the World Building articles here now: