@G Bemain: unfortunately I can't be in *any* way precise; I tend to be very much a stick my thumb in the air and guess type and I certainly haven't done any formal measurements.

I'm trying - and failing miserably - to find some examples. The search engine is not helping me... but at metered exposures of around a minute I've given it four or five, and for really dark scenes with a meter of four minutes I've given half an hour.