Wow, the article supposes _much_ more time is 'bought' than I do.
My rule of thumb is if I'm finished consuming my purchases (consumed at a 'reasonable' rate), I can stay as long as the coffee shop has at least one other free table.
This makes a lot of sense to me as a guard against being a complete jerk, but I tend to think that even then you should probably be buying something periodically.
Particularly if you work in a coffee shop because it's what you prefer, you should be paying a 'fair' rate for it, which I really don't think can possibly be less than $5/hr or so.
A coffee shop I used to work at semi-regularly [1] had a sticker discreetly placed next to the power plug, under the table, requesting that you spend around $5/hr if you're going to stay for an extended time. Seemed like a reasonable rule of thumb. I also like that it was done more as a "friendly reminder" / request, not in a heavy-handed manner, like the places that give one-time-use wifi codes that expire every hour.
I guess I might frequent busier places - I don't think that with my 'rule' I often go over your limit either.
Having said that, if I were a coffee shop owner, if the place felt empty I would rather have people sitting in it even if they're not continually buying, just to keep up the atmosphere.
My rule of thumb is if I'm finished consuming my purchases (consumed at a 'reasonable' rate), I can stay as long as the coffee shop has at least one other free table.