Formatting uptime as a human-friendly time interval
Bearing in mind that a linter can only look at how the code is written and not at what it's actually doing, switch statements (or the equivalent case as with Ruby) that don't do much are completely fine. 

A switch that tests many conditions is bound to be long-ish, there's not much you can do there. Every one of the warnings you get is a direct consequence of the fact that you're testing 8 values.

I would say this is one prime example of a situation in which you just don't waste your time in an attempt to make the linter happy. Extracting methods is going to clutter things up and look dumb.

The only viable option that I can think of right now is sticking those ranges along with their readable description in some kind of hash, and retrieving the value from there. But it's a compromise, really: if this method gets called often, you could be instantiating yet another object at each call.

I say you just ignore what the linter says in situations like this, or adjust its default settings to make it less bitchy.

Your method is fine, the intent is very clear to me, and it's not doing anything complicated. Don't get all caught up in an attempt to please a tool that doesn't understand your code to begin with.