The seven and half years of Shani will come once in thirty years. That means 25% of 30 years is bad if we consider that period as bad. In 60 years we will have two-time Shani and which will be 15 years. If really Shani is bad we have to suffer 25% of our lifetime. So I am not thinking that 10 years maximum is having no logic.
But it is not correct to think that Shani is always bad. There are people who did wonders during those seven and a half years period. There are other planets also like Kuja, Rahu and Kethu and some times these planets are also malefic. I feel we can't say that challenging period will be just 10 years in the lifetime of a person.
Knowledge is power.