First off, 7 years is the average life expectancy of a Smart AI.
Secondly, while most of your info is correct, Cortana is one of two known AI's created from a flash-cloned human brain, the rest weren't done from flash-cloning, the humans they were created from died as a result. Halsey used Flash-cloning because she needed a Smart AI and also needed to continue her work, so she cloned her own brain several times until she found one she could use for a Smart AI.
Also, Cortana's Life Expectancy was likely reduced by the amount of information she absorbed from the Installation 04, however she's still not entirely rampant, even after about 8 years. As read in one of the books [don't ask which, i can't remember] it's mentioned how during an attack on a Covenant fleet, she used copied forerunner code from a Covenant AI, to, along with the ability to translate Covenant language, create copies of herself that, while imperfect, allowed them to succeed. Using forerunner code and data Halsey removed due to her early stage rampancy symptoms, i believe she also managed to delay rampancy and increase her Life Expectancy to it's original 7 years or perhaps even further.