The SNP's White Paper ahead of the 2014 referendum said an 18 month transition was a "realistic timetable".
But Patrick Harvie said he believed an independent Scotland would need to set up its own currency.
He said the complexity of this could make the transition to independence a "longer-term project".
Mr Harvie, whose party supports independence and has called for a second referendum to be held, was speaking during a question and answer session organised by the David Hume Institute.
He was asked by an audience member how he would square his desire to tackle inequality with the "huge increases in taxes or massive spending cuts" that the questioner said would be facing an independent Scotland.
Mr Harvie replied: "I think one of the most essential things for the pro-independence movement to come to terms with is that to set out a proposition of Scottish independence that is based on a lack of ability to make our own macro-economic choices is almost to pretend that we can run an economy in the way that we run a devolved government - we can't.
"That to me has always required the movement towards the creation of an independent currency, and I accept that might be a longer-term project.
"It might be a 10-year transition rather than the 18-month transition that the SNP proposed in 2014."
The White Paper proposed a currency union between Scotland and the rest of the UK after independence - something that was ruled out by then-UK Chancellor George Osborne.
Since then, several senior figures within the pro-independence movement have said they believe an independent Scottish currency would be the best option.
The SNP last year tasked its Growth Commission with examining the economic options for an independent Scotland, including the question of currency.
But pro-UK parties have argued that a separate currency would incur additional costs, such as the potential creation of a central bank, and create more uncertainty for people and businesses.
Mr Harvie said he believed a separate Scottish currency was the only way of allowing the country to move towards an economy that was "able to generate the kind of investment that is needed in future prosperity".
He said the alternative was the "chilling prospect" of leaving those economic decisions in the hands of "those who are already making the wrong choices".
Mr Harvie added: "I think my own support for independence has always been as much about the opportunity to make choices that I don't believe the UK will ever make.
"Something like oil and gas, for example.
"I believe than an independent Scotland would find itself absolutely forced with the urgency of breaking its dependence on the North Sea oil and gas industry, simply because it can't last forever and simply because we are over-exposed to an over-valued commodity and an over-valued industry".
Full independence for Scotland could take 10 years to achieve after any future referendum, according to the co-convenor of the Scottish Greens.