The Bank of Japan will probably need at least nine years to normalize its balance sheet in the earliest-case scenario after a massive monetary easing program that has run for more than a decade, according to a former executive director.

"The hurdle is very high” for the normalization process, said Kenzo Yamamoto, the former BOJ executive, in an interview Monday. "The BOJ’s balance sheet has changed dramatically. It’s something I’ve never seen anywhere else.”

The BOJ is widely expected to kick off its normalization process this month or in April by ditching the current negative interest rate. After that move, which would be the first hike since 2007, how quickly it plans to tackle its bloated balance sheet will be a key question for investors.