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The simple answer is when they got the NFL in 1994.
You could answer also, when they went to a 7 night schedule in 1992-1993, which coincides with 90210.. Perhaps not the first 'hit' show for the network, but the first outside of sitcoms like The Simpsons and Married with Children
You could also put it much later.. More towards 2005 with American Idol being the first Fox show to be #1 for a season.