"So now we're on four rounds of chemotherapy, which should extend my life somewhat, but somewhat uncertain.
"It could be a matter of a few months; it could be over a year. Without the chemotherapy, they told me it'd be six to 12 weeks."
During a 30-year political career with the Labour Party, Cullen rose to be Helen Clark's right-hand man in their three terms in government from 1999 to 2008, as minister of finance for nine years and deputy prime minister for six.
He was the architect of the now-lauded retirement savings plan KiwiSaver, which was launched in 2007.
He has held a number of roles in the civil service since retiring from politics in 2009, including as chair of the Tax Working Group and Earthquake Commission, and deputy chair of New Zealand Post. Until this week he was chair of the Bay of Plenty District Health Board and a member of the Lakes District Health Board, but has resigned since the diagnosis.
"It is clear to me I will not be in a fit state to carry on all that I have been doing in recent months," he said.
"I have already stood down reluctantly from my long-held position as lead claims negotiator for Te Kotahitanga o Ngāti Tūwharetoa. The only major role I will maintain in the meantime is as chair of the Earthquake Commission (EQC) to see it through the release and the response to Dame Sylvia Cartwright's report into EQC and the Christchurch Earthquake sequence. I expect to make a decision shortly about the timing of my departure from that position."
Sir Michael started chemotherapy this week, and can be treated nearby at Whakatāne Hospital.
"I know what the end point is, so it's a matter of trying to enjoy life ... resting up, spending more time with family and friends.
"I've got certain goals in terms of that life span: One of them to be well enough in July to go with Anne [his wife] on a birthday holiday for her to North Queensland; the second is to survive to the general election, hopefully to see Jacinda [Ardern] elected; and the third is to survive until the American election in November, hopefully to see Donald Trump defeated."
He was knighted in 2012 for services to the state.