Depends on the market. 1 year stints in SV don't even register on the radar but in some European countries it could be a red flag.
Before SV I lived in Denver and any job tenure less than 2 years was a huge red flag, and even less than 3 would raise an eye brow.
If you've got 15-20 years of experience and you've never stayed somewhere more than 12-18 months, it's a very different scenario than someone with 5-6 years, even in SV.