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I would not worry too much about that as an exposure time. I recently had HA flats where I put too many folds of white cloth over the flat panel (An old habit from the SCT which can apparently get reflections between the corrector plate and the panel, probably not much of an issue with the Newt) and the exposure time was nearly 30 seconds. They worked out fine.
When you look at the histogram though, make sure no stretch is being applied, this is the histogram of a flat from my 2600MC with no stretch and a mild stretch. The separation between them disappears in the APP "Background calibration" step.
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