It's subjective and depends on the field. Software engineers tend to move around more frequently than say, petroleum engineers, probably owing to the larger number of companies who they can potentially work for. For me, looking at a software engineering resume, ideally I'd want to see at least a few stints lasting two years or more, but I'll forgive occasional short stints, since even great engineers can be bad fits in some companies.

I tend to look at this from a Bayesian perspective. If someone has left a lot of jobs after short stints, then that's a red flag. There's a possibility that they just got very unlucky (e.g. they are an amazing worker, but 5 companies they worked for folded within a few months), but it's also possible that they, for example, don't get along with people or lose interest in things very quickly. The more of a discernible pattern I see, the more likely I am to conclude the latter.

I might decide to hire that person anyway, but it's probably something that's going to weigh against them in the hiring decision.