This is accurate. The best I ever saw lasted about 2 minutes, after hours of sitting watching basically nothing. Everyone else had long gone inside but I still had my camera set. Absolutely worth sitting alone on a hillside, it was by far the most amazing thing I’ve ever witnessed.

It also helps to let your eyes accustom to the dark, IIRC 45 minutes is ideal.

The boilerplate number in the UK is 76%, but this is referring to higher-risk instruments.