insy i would imagine that the pads would heat up pretty quickly, so there wouldn't really be a risk of putting them on for too long. However, the usual rule for a sports injury would be 5-7 minutes for the first 48-72 hours then 10-20 minutes thereafter.
Out of interest, the reasoning for this is that if you apply for 5-7 minutes, you limit the blood flow and thus the inflammation. When you hit 10 minutes, the body's survival instinct sets in and you undergo alternate increased then decreased bloodflow, creating a pumping effect, thus increased bloodflow.
For episiotomy, i would imagine that you are really using the cold to numb the area, rather than being particularly therapeutic. I would stick to the 48-72 hours of this and then use heat, vitE cream etc to try and get it all moving and sopt the scar from contracting too much. (after 48-72 hours primary healing has set in and you want movement to encourage further healing, but also to prevent shortening- imagine it like you have cut yourself on a knuckle- you have to keep moving it and cracking the scar)