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I'd suggest you can be quantitative about this if you like.
Assuming you've chosen an exposure time so that you are sky limited with respect to read noise, then it would make sense to ensure that the noise from the dark current is of the same order (or less!) as the (now negligible) read noise.
So if your camera's read noise is R and you are exposing for T seconds, you need to cool so that your dark current is no more than R^2 / T.
Typically for CMOS cameras at reasonable gain, R is say 1.5 ish. So you might try to get the dark current down to say less than 2/T.
For the ZWO2600, the dark current is remarkably low and is about 0.002 at 0 degrees apparently. So even at 0 degrees I'd suggest you could do 1000 second exposures.
For the ZWO1600, things are much worse, and say you want to do 10 minute narrowband shots (T=600), so you need dark current no more than about 0.003. This is not achievable even at -20 (may be at lower temps, but the curve I saw didn't go down to -30, which is apparently the lowest it can be set). At -20 degrees, you get dark current of 0.0062, and so you can do no more than about 300 second exposures before the dark current noise climbs above the read noise. That's not to say you CANNOT do longer exposures, just that your noise from dark current will start to be the thing you worry about rather than your read noise.
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