The stainless steel njoy plugs are really comfortable for long term wear (over night, in public, etc.).
The biggest sore point for me (pun intended) is the shape of the flange and how well it sits between the cheeks. Round flanges tend to irritate me the most, and anatomically correct flanges (long and narrow) seem to fit more comfortably. Some people really like the Tristan plugs, but I find it has the worst flange for long term wear. I trimmed the pointy corners off mine (I also trim the sides off round flanged plugs), and it was better.
Never had any pinching/irritation from the njoy plugs, though.
Anyway ... back on topic ... if it hurts (in a bad way ;)), its harmful. If it doesn't hurt, you can go for as long as you like. Let pain be your guide.