Healing Potions heal for 150 health over 15 seconds, i.e. 10 health per second
Champion regeneration is about 5 health over 5 seconds depending on the champion, i.e. 1 health per second
Ignite does 50 + 20 per level over 5 seconds, i.e. 10+4 per level per second and reduces healing by 50%.
That means if you are at 1hp a health potion won't save you at any level with a reduced healing of about 5-6 health per second compared to 14 damage per second but you can survive it if you have more than ~40hp on level 1.
Healing Potions against ignite are more of a last straw, not a reliable way to counteract it.