Hi DavyLP,
It all depends on what you are trying to accomplish with this Hydro-therapy!

If you are physically able to do a full body soak in a tub or a hot tub for those with access to one, then the time limit will depend on how strong you are!

What I mean by this, is that if you are loaded with Toxins, then you may only soak for a short time to begin with in the hot water and if you are using Epsom Salt and Baking Soda and Hydrogen Peroxide in this water, these make the soak very much more powerful and this will draw Toxins out of the body and may completely drain your body of energy and overcome you , if you soak to long at first!
Then when you get out of the hot soak ( it is important to keep the water as hot as possible during the soak, as this gives max,. benefit ) then you need to take a cold shower just for a little bit to take the heat out of your body, so as to convert the nitrogen isotope!

Now if you are able to get in and out of a tub for a full body soak and may stay in the hot soak longer, then a 30 minuite time is sufficient time to really work your body in the hot soak, then you take a cold shower to convert the nitrogen isotope for to cause the body to become Alkaline overall in the Lymph system!

But if you , say have feet and leg problems and maybe have a Weak Heart, then you may do a foot soak with the same ratios , yet maybe only one fourth of a LB of the Epsom Salt and Baking soda and a small amount of Hydrogen peroxide in the foot tub for 30 minuites!
Just soaking the feet in this by it's self with out the cold water makes a hugh difference!

But if one was to switch back and forth from Hot to Cold , then one may get much faster results!

As to if a person has had a Heart Attack and has lost use of their arm on one side, they may regain the use again, if they have someone do this Hydro-therapy for them.
Example: If the therapist will soak the arm in a Hot soak for several minuites and then place the arm in ice water for just a little bit and then back to the Hot water for a longer time, this method may cause the arm to start working for the person again, if it is done properly and for long enough time(it may take weeks or more to to accomplish all one wants to accomplish)!

But a person also needs to change their Diet, and eat foods which are Alkaline forming , so as to provide the proper Healing for the body!

Smile Tis your choice.