This image was shot on a 35mm camera, at 10pm, and metered normally for the box speed of ISO 400. 1/8 sec f/1.4. The focal length was 35mm, but I don't think I could have gotten it without support: I had my elbows firmly placed onto a pier rail.
Without support, I find the 1/focal-length rule to be quite reliable for me, with decent success rate at double that. But that's only true for 35mm format. My handholding record is quite miserable with MF cameras.