It's definitely bad etiquette to tie up a table without ordering anything, but I think my sense of "long period of time" differs from yours. I think it's okay to tie up a table for an hour if you've ordered something in places where it's otherwise pretty standard to be somewhat leisurely.

I agree that when a place is obviously quite busy and people are unable to find a table because most tables are in use by those who haven't consumed food or beverage at the establishment for a really long time it's bad etiquette to just keep sitting there for hours tying up a table.