How Long To Cure Allergic Dermatitis?

I was on hydraquinone cream. I developed red, blotchy and itchy rashes over face after sunexposure. Consulted our college dermatologist, he said it as irritant/allergic dermatitis, and prescribed venusia cream at day times, desonide cream at night and levocetrizine tablet at night!
I wanna know how long will it take to cure my skin and what all precautions should i take.