The Renewable Heating Incentive (RHI) scandal could cost the Northern Ireland taxpayer as much as £490m.
The inquiry's chairman, Sir Patrick Coghlin, said there was a "huge amount of material" to be obtained and considered.
He said the material must be gathered before public hearings could begin.
Sir Patrick said: "I cannot, at this early stage of the investigation, give a date by which our work will be complete.
"At present I can say that it will not be possible to report within six months."
The RHI scheme was set up in 2012 when DUP leader Arlene Foster was in charge of the Department for Enterprise, Trade and Investment.
Its aim was to increase the creation of heat from renewable sources, but businesses receiving more in subsidies than they paid for the renewable fuel, and the scheme became majorly oversubscribed.
The fallout from the scandal resulted in the resignation of the deputy first minister, Martin McGuinness, the collapse of Stormont's institutions and the calling of a snap elections for 2 March.
Sinn Fein has stated several times that it would not support Mrs Foster's nomination as first minister until her role in the debacle is clarified by an inquiry.
The inquiry was launched in January. Former permanent secretary for the Department of Heath Dame Una O'Brien will join Sir Patrick on the inquiry panel.
Earlier this week, it was announced that inspections of RHI scheme sites would begin in May in an attempted crackdown on abuse of the scheme.
The public inquiry into a botched green energy scheme will take longer than six months to conclude, its chairman has said.