What you need for the conversion of collagen is a certain amount of energy. It is a complicated process - the melting point is around 70°C for the type of collagen contained in beef, but the melting does not happen instantly once the meat reaches 70°C. In a pressure cooking, you can apply the same amount of energy in a shorter amount of time. This is not bad, as opposed to slow roasting of collagen-poor meat.
In collagen-poor meat, you have two types of protein, which are soft and wet. Under heat, they curdle, becoming tough and dry. The perfect meat is when the first type has curdled (so the meat is not raw) but the second hasn't, so it still holds juices inside. If you curdle both, your meat gets tough and you can't take it apart with your teeth.
In collagen-rich meat, you curdle both proteins - the collagen itself is tough and you want to melt it, but this happens long after the meat has curdled. But because the muscle fibers are not clinging to each other, but separated by collagen, you still get tasty meat. For that, you melt the collagen into gelatin, and serve the meat warm, so that the dry fibers are separated by the smooth, juicy melted gelatin. Unlike slow-roasted meat, you don't have to tear the juiceless fibers apart, and the gelatin makes up for the missing meat juices which were expelled from the cells during curdling.
So, in slow-roasted meat you don't want to cross the temperature limit for curdling a certain protein, this is why you have to apply heat slowly until the center of the meat has cooked, without the outside getting overcooked. In collagen-rich meat, there is no upper limit at which the meat gets non-tasty, so you can push the energy needed for the collagen-to-gelatin conversion quickly into your meat. The pressure cooker can do this better than the normal boiling process.
Best Answer
This depends on the end result you are looking for. At some point, if you leave it in the bath too long, the texture will probably go too mushy for most peoples' liking, but here are a few observations: Baldwin states that 131F (55C) is the lowest temperature for collagen conversion, and that at higher temperatures the denaturing happens more quickly. So, you could increase the temperature. Since you cooked at 55C, you probably want it on the pink side. You could go as high as 57C or 58C (though now we are getting into "medium" territory) and still have pink meat. Chefsteps has a recipe for chuck roast, and provides a time and temperature guide. They cook for 18 hours with temperatures ranging (depending on your desired outcome) from 54 - 72C (129-162F). I am guessing that 18-24 hours produces something that resembles a more traditional roast in the end. If you are looking for a softer texture, I would go slightly higher on the temperature, and longer on the cook, maybe even 48 hours.