Labor MP Joel Fitzgibbon has predicted that the royal commission on child sex abuse would be a ''big and slow moving beast'' and could take 10 years.
When announcing the royal commission on Monday, Prime Minister Julia Gillard did not put a time limit on the inquiry, saying the timeframe was ''not knowable'' but that it would take ''some time''.
The terms of reference are due by the end of this year.
On Tuesday, Mr Fitzgibbon described the inquiry as ''a big and slow moving beast'' that could take a decade.
''Make no mistake it will cause trauma for many individuals and organisations,'' he told ABC Radio.
But independent senator Nick Xenophon said a deadline of two years should be imposed on the nation-wide inquiry into child sex abuse in churches, charities, state governments, schools, community organisations and the police.
Senator Xenophon, who had been pushing for a royal commission, said the inquiry is a once-in-a-lifetime chance to get it right, provided it has the appropriate resources.
''It can't drag on for nine or 10 years as the Irish commission of inquiry did,'' he told ABC TV on Tuesday. ''A two-year time frame seems appropriate.''
Shadow attorney-general George Brandis noted that there should be an indicative time limit.
''But these time limits can't really be enforced in a strict way,'' he told Sky News.
''If the royal commissioner uncovers avenues of inquiry that need to be explored, well they have to be explored.''
Trade Minister Craig Emerson said it was important that time limits were not imposed on the commission.
''There will be many people who will want to come forward and a lot of investigation to do because this has been a scourge in Australia for a very long period of time,'' Dr Emerson told Sky News.
''It will take time to get to the bottom of it.''
Mr Fitzgibbon represents the Hunter region, which is currently the focus of a NSW government inquiry into abuse by Catholic priests.
He said that the NSW investigation - announced by Premier Barry O'Farrell last week - should not be put aside now that the national royal commission had been announced. However, he said, it could "move quickly".
''It was quite specific,'' he said of Mr O'Farrell's inquiry.
Mr Fitzgibbon added that supported a royal commission because it was the only inquiry capable of compelling witnesses.
''I still believe its the only way to provide closure for the victims,'' he said.