I think it was down to around 3 million viewers (give or take a few hundred thousand either way) by it's final season on UPN.
Granted, the current TV output of the successor network The CW also suggests that Star Trek would not fit it's style of programming but there have been shows getting by for years on ratings less than even that on it the last couple years (Supernatural, Nikita, etc).
'If the Apocalypse starts, beep me!' - Buffy Summers
'The sky's the limit.....' Jean-Luc Picard, 'All Good Things'
courtesy of Saquist