If you show that you changed jobs at a rate of 6 months then hiring managers will extrapolate this and conclude that you are less likely to stay longer than the preferred minimum 12 months. A severe "job hopping" alarm will be triggered in this case.
If you change jobs every 12 months, then you're in more-likely-to-land-an-interview territory, but there will be questions and reservations, depending on the job position.
Anything longer than that and it's probably unlikely you'll trigger job hopping alarms for engineers. PMs may be a bit different depending on the industry. For example, if you're a PM in a slower moving technology industry like telecom then anything less than 24months would probably count against you, especially if you're moving between different completely markets.
Startups help to explain short job durations, but hiring managers would then expect to hear what you learned and convince them you were insanely productive during said period.