How often should you use Light Treatment safely...

I have suffered from guttural psoriasis on my limbs and trunk and scalp psoriasis for many years and tried every cream going. I have recently hired a 9 Lamp UVB Narrowband Phototherapy Unit for psoriasis on my body and I have to say it is working wonders - I cannot get over the results. However, I was wondering how long it is safe to use these as I was considering buying one but have read varying things about how often you can use the light treatment... can anyone advise ? Best wishes