The idea about a 30 second duration would be for a larger boost to a ATT up %. Where I would think longer duration would come with a smaller % boost. When I start to consider a hypothetical spell, I try to take into consideration pro and cons of it. And for my preference I'd rather deliver a larger boost for a shorter time the a smaller % for a longer duration. But this difference in opinion can be settled quick with 2 tier version's of a spell like this. One that last longer and give a smaller over all %. And one that is shorter for a larger %.

I also understand that the spell could just scale with enhancing magic value. And that would work too. But honestly where do you think that would more likely to land? In the percentage scale 10 to 12 percent boost or 25 to 30 percent. In the land of not over powering RDM, I doubt we'd see the higher end.