Why on earth would you ever do this? If you don't want anything living in your medium then grow in coco or hempy style.
If you want to kill ever living microorganism than just nuke it for 30seconds. Make sure the soil is moist and put a peice of clingfilm over the top.
When i experimented with this i microwaved it from 5-15 mins. I noticed around the 7 minute mark it started to steam in a 1k watt microwave. I did it because the soil had root aphids. Killed them but eventually just cooked it in oven after reading a lot of the root aphid thread at icmag.
180f , moistened soil in conventional oven for 1 hr. Kills weed seeds & soil pathogens. An opened bag of soil can become contaminated sitting around in your garage or basement. Dormant bacteria or fungus spores are often air borne. Temps over 200f starts killing nutes.
Here's why he asked. http://forum.grasscity.com/sick-plants-problems/1336338-damping-off-troubles.html
I would just get yourself some different soil, fuck putting it in the microwave. I only put food in there. Not for dirt.