This site (
www.thehealthyhomeeconomist.com/castor-oil-pack-detoxification/) says 45 - 60 minutes, but they aren't aiming this at those that are so overloaded with toxins. So you want to start off much slower, say 15 - 20 minutes and see how you do. If it causes herxing, reduce the amount of time on the castor oil pack until you no longer herx.
For starters, I would say do this no more than twice a week. You can build up to doing it more often as you go along - but if you herx, back down on the amount or the length of time you spend on the pack. It means that there are too many toxins in your blood stream for your body to deal with and it's telling you about
it. Slow and easy is the way you want it.
And be sure to keep doing other things to help remove those toxins like the epsom salt soaks like you did! Dry skin brushing is also good, as is oil pulling, FIR sauna if you have access to one, and other things.