@Ju_Jitsu_X That'd be cool, although I suspect that the reason they went with a percentage is because they wanted to make it seem more like your character was charging up for a cool ultimate move rather than something that they can do every minute.
Also, ult accelerants charge like 35% no matter the ult, (unless it's Wattson). This would make it slightly difficult to translate that into time. (Not that much more difficult.)
Great idea though, this should be put in the game as an option.