When doing a hazard analysis, are there any guidelines regarding severity of consequences? What if the release was of 500 mL of hydrogen?

Frequency and severity off consequences are situational and subject to the safety review team’s best judgement. One measure of severity is an estimate of the energy released if ignited. Assuming the worst-case mix to be stoichiometric, the energy content of a 500 mL of hydrogen in air is about 0.2 Wh (700 Joules), comparable to the energy release of a wooden, blue-tipped matchstick (~1kJ or 1 Btu). This may not be very significant in a safe location such as an operating fume hood. See Risk assessment and risk ranking at H2Tools, Best Practices: Ranking Risks, for more information.