That would depend on what is causing the itching. Try shampooing daily with a shampoo with zinc pyrithione in it (Head and Shoulders and generic knockoffs seem to work equally as well). If that works, then you can usually cut back after that, to once or twice a week. That seems to work for me.
I am not a doctor, nor have I ever played one on TV...
I have Hashimoto's.. and Graves. I get periodic flares of skin inflammation and itchiness. I see a dermatologist and allergist about it. It is an autoimmune response. My Dermatologist has me on elidel and metrogel for the face (prescription only) and a dual steriodal cream coctail for my body. I also am told to use Head and shoulders OR a dandruff shampoo as my bath soap. Meep is giving you solid advice there. When the issue gets excessive and my eyes swell shut and I begin to have respiratory issues I have to take prednisone to make it back off. I am due to allergies and vasomotor rhinitis on 10 mg Zyrtec daily. This now comes in generic OTC form.
When I have a flare I have to avoid all allergen foods and do my best to avoidd all contact allergens. Lots of lotion.. steriodal lotions are even better.. and my antihistamines do help. the degree of the relief will depend on the source of your itch.
If we learn by our mistakes, I am working on one hell of an education.