It depends of the kind of equipment that you use to make the determination, usually if you use a moisture content analyser, you can have your result in minutes, but they are not cheap
If you apply something like 100 °C for 3 hours is because you have an oven, and after the 3 hours you have to place the sample quickly in a
desiccator.
I am sorry that I don't have the SOP but I think that you should keep in mind the conditions that you have because they will determine the SOP, with the moisture content analyser you do very little, usually you should take the same size of the sample, put it in the equipment, which should be be previously calibrated, before you start to operate take down the weight of the sample, many equipment do this for you too, so you only put the sample on the plate, close the lid, press the button and wait for the beep, and you'll se the result on the screen. Usually you can have different programs. Check for Mettler Toledo moisture content analyser and you may find and SOP that suits you.
I wish I could be of more help.
Even I don't have a written SOP for the traditional method which is more likely to what you describe. I could give some ideas about it considering that I have used in many occasions even though, slightly higher temperatures and different samples, but all is based in weight difference of the sample before and after.
Is important to make the determination twice and if the difference is considerable it means that something is wrong. This usually can be solved with a third repetition.
Best regards