The amount of fan time required depends on how much moisture needs to added and the evaporation rate. Evaporating water with electric heat is expensive caompared to heat from a heat pump or high eff. natural gas. One KW of electricity is needed for 3 lbs. of water evaporation. Using heat generated by a heat pump at cop of 3, one KW is needed for 9 lbs. of water evaporation.
Consider that excess air leakage causes low humidity, better to air tighten the home to avoid extra heating and humidifiing cost. But if you must humidify use Skuttles air evaporating humidifier. It will save a bundle. Regards TB
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"