How long you should steam depends on a couple factors.
Steaming is very powerful and some women only need to steam for a short amount of time (10 minute mild setup) in order for it to be effective. If they steam for too long it might be overdoing it. The women that fit into this category have short menstrual cycles (27 days or fewer), intermittent bleeding or have signs of excess heat (hotflashes, nightsweats, infections).
Women with normal menstrual cycles (28-30 days) or missing periods usually benefit from a steam session that is 30 minutes long on an advanced steam setup.
Some women have advanced issues that are higher up in the uterus. For these users, steaming for a long session is most beneficial.
Use the following guide to determine the duration of your steam session:
Short Menstrual Cycles (27 Days or Fewer)
Bleeding In Between Periods
Normal Menstrual Cycles (28-30 Days)
If you are still not sure how long you should steam it would be best to work with a practitioner.