If you started a Mage character, and you did nothing but Necromancy spells, used them every opportunity you got, within a few weeks you'd have skill enough with those spells to be able to go out and pull your weight in a group of veteran characters. But you'd have a very limited set of things you could do (cast DoT's and some debuffs, basically). If you wanted to do more than that, you'd have to branch out, set your Primary skill lock on Necromancy so it wouldn't decay, and then work on something else.