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John, each sub has a fixed read noise that can't be removed (the bias signal can). The fewer subs you have for a given integration time, the lower this noise relative to signal. So, in an ideal world, you'd want your subs as long as possible to minimise the % contribution of the read noise.
In the real world, long subs are problematic due to need for excellent tracking, avoiding wind/satellite/plan trails, bumps etc. They are also a problem because sky background will accumulate to a point where you start losing dynamic range.
Paul was imaging here in narrowband in a dark site, so sky background will be minimal. Thus, provided he has tracking and scope protection under control, longer subs are desirable.
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