About 4 months from memory.
It got to the point where NBN News outrated it in Tamworth .
As for the history of Prime Local News, here in Newcastle, it was launched with much fanfare in February 1993, with former NBN stalwart Murray Finlay as the main anchor. Janette Hankinson (who I still don't know much about) was also a co anchor. Former Knights player Mark Sargent presented sport for a while too.
Prime's studios were former squash courts I think in Newcomen Street, Newcastle, only a couple of doors down from the ABC.
Finlay lasted about a year, and Brett Lavaring took over the presenter role in 1994 I think. He had previously presented North Coast news out of Tamworth (I think David Evans still did the North West bulletin for a while before he retired, as he presented in the 9&8 Television days too).
Also in 1994, Prime launched "Prime Weekend News" out of Tamworth in response to NBNs popular weekend news across all of Northern NSW as a composite 30 min bulletin. The middle block was separate local/sport news for each sub market. It was hosted by Kylie Gillies, and it only lasted about a year as well.