I know that burning enemies take extra damage from after burn, but how much damage does after burn deal?
From looking at the Creation kit, the fire spells have 3 attributes that determine after burn damage.
Interestingly, all spells except Wall of Fire have the same taper weight, .3. Wall of Flame has a taper weight of .5.
There is some curve that we can use to describe the after damage of each spell. For most of them, it's going to be 30% of the damage done, decreasing over time by some unknown (but constant across all spells) amount.
The burn duration of each spell is listed below:
Through my experience and no other sources I would approximate the extra damage done is 40% of the spell as a damage over time that starts after you stop using the spell. Many people have reported using Flames on and off will do more damage.
It seems that you do 12 damage per second and release, afterburn takes effect and does 5, and then immediately after you stop Flames you start again, so that you are doing the regular flame damage while your afterburn is in effect, then release and start another afterburn effect. Other people and I have experienced the enemies health bar go down significantly faster than just holding down Flames continuously.