Well, obviously different brands offered can have different properties, but in general, the initial CCL4 activity for most commercial carbons is about 60%. Some manufacturers use reactivated carbon (carbon that has been previously used, but reactivated by a high temperature process which results in a lower activity, around 50%), or a blend of virgin (meaning it is brand new, never been used) and reactivated which can be anywhere in between.
In addition, the material the carbon is made from can make a big difference. For reef tanks, you really should use bituminous coal based and stay away from coconut shell, lignite or wood based carbons. Most brands I've looked at in stores is already bituminous coal based, but I have seen some coconut shell and others out there.
So I would guess in my system, a typical commercial carbon with a starting activity of 60% would be spent approximately 20% faster. Think of it this way, the starting activity of 70% minus ending activity of 20% provides 50 usable activity points. Starting activity of 60% is 10 points less, so 10/50 = 20%. Not exactly right, but for all practical purposes close enough.