How can I track "Magic Weapon"s cooldown?

The Wizard's magic weapon states that it increases damage by 10%, lasts 5 minutes. However it doesn't give a duration timer near my health orb like similar buffs.

How do I know when my magic weapon has expired?

Magic weapon (and all its runed varieties) has a pretty distinctive glow, and when the glow fades, so too has the buff.

But you shouldn't even need to keep track of it. Magic Weapon costs a measly 25 arcane power, which is 2.5 seconds worth of regen or less! I think the real solution is to get in the habit of casting Magic Weapon at the end of every fight, because there's no reason not to refresh the duration.

Between frequent recasting and the buff's 5-minute duration, you'll never have to worry about letting the buff drop.

And for the future, the 1.0.3 patch will add in a buff timer (point 7).

We're adding a buff icon for Magic Weapon in 1.0.3. We don't intend to add one for Familiar as it's a physically visible object on your screen. Magic Weapon was actually impossible to tell if it was up in quite a few cases. We'll be continuing to watch skills and buff icons, though, as there could be some situations where we need to expose more.

You can keep an eye on the weapon (it should be blue as long as it's in effect) but otherwise there's no timer than I know of.