Well this is a really good question and to be honest, there isn't a simple answer to give as everyone responds differently.
Below is a table showing the suggested session frequency for respiratory conditions that the World Halotherapy Association have shared.
With regards to skin conditions such as Eczema and Psoriasis, most people have started to see improvement after 3-4 sessions (ideally done within a week) and after 2-3 weeks are able to start reducing the frequency of sessions down to a 'maintenance' regime that suits them.
Some people are able to go several months without any sessions until a flare up occurs, however this is completely dependent on each individual, the triggers and the severity of their condition.
The key with treating skin conditions is to try and let the salt remain on the affected areas for as long as possible before washing off (or showering), ideally overnight.
Please remember that everyone responds differently and you may experience results sooner than expected or it may take a bit longer.