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If I recall, it is around 3-4 magnitude in a 60 second exposure, depending on CCD sensitivity. In a Canon DSLR I get to about magnitude 17 in 60 seconds on the FLT98, with a Canon 40D, vs the manufacturer claim of 14.5 visual, for a magnitude gain of around 2.5. The sensor is 37% QE on green light, with an RGGB bayer matrix, so it is 18% efficient on green. A good CCD would be up to 4x that efficiency for purely luminance exposures, so I would be pushing magnitude 18.5 in that same time, or a gain of 4. Filtered, of course, I would still end up with QE loss and lower net gain.
One magnitude is 2.512x brightness, so doubling your exposure time will yield slightly less than one magnitude gain. A 6 minute exposure should go about 2 mag deeper than a 1 minute exposure, if I have done my math right. 10x the exposure will give you 2.5 mag gain. As you can see, it is a diminishing return.
Regards,
Eric
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