The phrase "asserted time and again" is correct and usable in written English.
It can be used to emphasize that something has been repeatedly stated or claimed.
Example: "The scientist asserted time and again that climate change is a pressing issue that requires immediate action."
Alternatives: "repeatedly claimed" or "stated over and over".
To the outside world America asserts time and again that the sanctions apply neither to food nor medicine, and that it is in Mr Hussein's power to increase their supply dramatically (on the eve of Kofi Annan's visit, the UN agreed to relax the cap on Iraq's oil exports, doubling their value to $5.2 billion every six months).
Anti-GMO campaigners, including many organic supporters, assert time and again that genetically modified crops have not been safety tested or that the research done to date on the health or environmental impact of GMOs has "all" been done by the companies that produce the seeds.
They justify this hypocrisy by asserting what has been disproved time and again — that tax cuts spur the economy and compensate for any lost revenue.
So whilst, time and again, asserting that they are not criticising German competitiveness, they want a change in German policy.
Time and again, Friedman has asserted the legitimacy and legality of the settlements, contradicting longstanding US policy.
While some experts said Mr. Bush had faced an even more difficult challenge than his predecessors because the five-week post-election stalemate limited the time he had to prepare -- a point the new administration has made time and again -- many others asserted that Mr. Bush actually benefited from the truncated timetable.
Specifically, the Japanese government asserts, "From 1885 on, our government conducted on-site surveys time and again, which confirmed that the islands were uninhabited and there were no signs of control by the Qing Empire".
An intelligence assessment drafted by the CIA months prior to the March 2003 US invasion of Iraq asserting that Saddam Hussein harbored an active weapons of mass destruction (WMDs) cache has been thoroughly debunked time and again.
