How long to stay in a role to avoid being considered a "job hopper"?

It really depends on the industry. These are opinions, but for a seat-of-the-pants dev job like a web developer, at least a year is fine, with the occasional longer stint of 3-5 years recommended so it shows that you are capable of having a little patience and can own the full product lifecycle.

For more intentional work in regulated industries, 3-5 years seems to be the minimum before things appear suspicious

Basically, if you just want to be a programmer/code-slinger and not aspire to more, then hop around all you want. If you want to be taken seriously as an engineer, then expect to serve time in a position for a while and work in the trenches with the other engineers.

I think you should avoid 2x consecutive 1y.