User blog comment:Lykrast/Double the spells/@comment-7802232-20131105104514

Twitch's  passive lasts for 12 seconds and stacks up to 12 times:

Minimum of 24 / 48 / 72 / 96 dmg over the entire duration

Maximum of 288 / 576 / 864 / 1152 dmg over the entire duration