The phrase "many many decades" means a long period of time, typically referring to several consecutive decades or multiple periods of ten years. Full definition
Each book is the result of decades of experience working with everyone from individual investors to large venture capital investment firms. (mystockmarketbasics.com)
Many of us had been training for decades for just such a call. (politics.co.uk)
These areas offer some profound opportunities, in our view — perhaps the best in decades in some cases. (us.beyondbullsandbears.com)