80-505 True damage across 5 seconds.
Lets say I'm playing any adc in the game, meaning that I don't buy health items. In this case, i'll use Varus, which has 2101 base health level 18, with his level 1 base health being 499.
Ok, lets try a mage. Orianna has 530-2077 base health in the game, which again, doesn't buy health items unless it's insanely situational.
Supports also have this problem, where their base health is so damn low, that they just get completely bum fucked by ignite. I don't know who's idea it was to give this damn summoner spell so much power, but it's complete nonsense and deserves a nerf. A summoner spell shouldn't do more than an abilities worth of damage, ever.
Also yes, I know fourth is spelled wrong, I didn't have room in the title.