Depends on the build, really. If you're planning on using a lot of summoned critters or alteration or whatever, magicka should be fairly high.
I have a build though, I call him an "arcane skirmisher," all he needs is a frost/flame atronach and bound bow, with the mystic binding perk. I use light armour and play on expert, but my armour level is only about 28 even though I'm level 25ish overall - purely because nothing gets close enough to hit him. 3x sneak arrows are enough to down most enemies in a few hits, I'm thinking of turning him up to master.
He has about 160 magicka, which at high conjuration level (think he's about level 65 conj), is enough to summon a bow and frost atronach, and I can't see myself putting any more into magicka unless I feel the need to summon two atronachs or use storm atronachs.
Here's his perks
Skyrim Perk Calculator - Plan the perks for your Skyrim character before spending them!
Hope this helps!