User blog comment:Emptylord/Season 3 - PBE Patch Notes/@comment-5170841-20121111055618/@comment-3974211-20121111140222

It takes 12 seconds for Blackfire to do 's base damage. With the 25 charge cap, it will only do this damage to up to 2 targets (if you're using AOE that hits 5 targets, you will run out of charge long before you were even close to using DFG). I'd say it's fairly balanced.