Im afraid its largely hardcoded. You have some degree of control over the scorches in FXList.INI by being able to specify the Radius = and IIRC you can specify the Type of scorch with these parameters but Im not sure if they all still work;-
SCORCH_1
SCORCH_2
SCORCH_3
SCORCH_4
SHADOW_SCORCH
RANDOM
That hardcoded element simply uses the EXScorch01 texture and applies some mathematical formula to it based on those parameters.
I cant recall any setting that allows you to control how long they last for though
