how many hours/days will it take to climb the whole Huashan mountain?

I think you may need six to seven hours.
The route can be like this: walking to Fish Stone – Five-Mile Pass – Stone Gate - Shaluo Terrace – Maonv Cave – Qingke Terrace – Mind Changing Stone (Huixin Stone) – Thousand-Foot Precipice – Hundred-Foot Crevice – Taoist Temple of A Group of Immortals – Laojun Furrow – Ear-Touching Cliff – Heavenward Ladder – Black Dragon Ridge – Golden Lock Pass – Five-Cloud Peak – Sunshine Welcoming Cave – East Peak.
Then you can climb other peaks.
For get to the bottom, it is suggested to take the cable car to get down.