Ideally, people should wash their hair no more than three times per week. This is due to the fact that shampoos contain chemicals that increase oil production of the scalp. So washing only a few times a week will keep hair clean and less oily.
How often you should wash your hair in a cold climate depends on your hair's natural greasiness and how comfortable you are with it. You should wash it whenever you feel like it is too greasy.
Never, it ruins your hair and kills it.
You should wash your hair every other day or less if the hair is thinning.
Twice a month :)
Everyday.
Most sources recommend that people should wash their hair daily or every 2 days. This is usually because if hair is not washed this often it can begin to look oily or greasy.
If you are straitening your hair but do not want to damage it, then you should use special protention products. Often you'll find specific shampoos dedicated to hair that is often heated (via straitener or blow-dryer), personally I would recomend Tresemme. However you should not straiten your hair too often.
You should wash your hair every week if you have thick hair and you don't always put oils in your hair. UNLESS you have thin hair. You should wash your hair hair every other day!
Every other day
Once a month...!
twice a week
Every other day should be enough