Not possible to say because each and every places of Himachal is heaven you have to give sufficient time to feel this heaven, otherwise you can cover basic Himachal in 3-4 days like Shimla and Manali.
There is a lot to explore in Himachal Pradesh. So, it totally depends on what places you want to visit. You can cover the famous places in about 10 days. If you want to explore the hidden gems, then you need to plan accordingly. https://www.tripoto.com/himachal-pradesh/tour-packages
It’s only apt that your you should plan your Himachal Pradesh itinerary for 10 days and spread it out for more than a week in order to capture as much as this enchanting region. For Himachal Pradesh itinerary for 10 days, you can go for this itinerary: Day 1- Shimla
Day 2- Kufri
Day 3- Shimla – Manali
Day 4- Manali
Day 5- Solang Valley
Day 6- Kullu Valley
Day 7- Manali- Dharamshala
Day 8- Dharamshala
Day 9- Mcleodganj
Day 10- Dalhousie – Khajjar https://www.tripoto.com/himachal-pradesh/tour-packages
I apologise to all the fellows who have posted their views about how much days are enough for the mentioned place and I would like to say that even one year is not enough to explore the whole Himachal Pradesh many of you guys will hate my answer but this the truth......