When soaking in an Epsom Salt Bath you should soak in the Epsom Salt Bath for 15 to 20 minutes to get the full benefits of an Epsom Salt Bath.
If you're trying to relieve constipation using an Epsom Salt Bath you should sit in the Epsom Salt Bath for 20 minutes at least to allow the Epsom Salt Bath to work.
After 20 minutes the Epsom Salt should fully work on the constipation and get your bowels moving.
I sometimes will soak in an Epsom Salt Bath for 30 to 40 minutes but no longer than 40 minutes at a time.
To make an Epsom Salt bath you should fill the bathtub with warm water and then use 2 cups of Epsom Salt for a Standard Size bath tub of water.
If you soak in an Epsom Salt bath longer than 40 minutes the Epsom Salt Bath can begin to dehydrate your body.