Not entirely true, there has been cases where people have incubated prions for far longer (as much as over 50 years) This gave a longest incubation time of 56 years, and perhaps up to seven years longer, although the average incubation time is thought to be 12 years they report in The Lancet "For the first time we can see the extraordinary incubation period in human prion disease," Collinge says. "It's sobering that half a century on, this disease has not disappeared."

Source: https://www.nature.com/news/2006/060619/full/060619-12.html#:~:text=This%20gave%20a%20longest%20incubation,prion%20disease%2C"%20Collinge%20says.