I soak my adults 2 time a week for 20 to 30 min. Water temp about what their habitat temp. About 78-85.
Just make sure it doesn't cover their chins and that they can get their little heads out of the water. Change water when they soil it.
I usually only do it once a week, but if I feel it's been hotter, drier, or if the food I am feeding that week happens to be lower moisture content, then I up to twice.
I have one Russian, my antique male who hates being in water. He is only forced to take a 10 min soak. A couple of the females really like soaking, so they get longer periods like 30 mins. The rest fall in the middle times.
Like said change water when soiled or if it's cooling down too much.
A sick tort or one not eating, I will give more soaks to...sometimes even once a day, just depends on what I think they need and if it does seem to help.
Of course, if you have a tort who is eating well, eating good quality food and uses his own water bowl, you might not even need to do a forced soak.