While I can't comment on using a pressure cooker, I can give a little info on when beef bones are "done" rendering their gelatine.
This is how I was taught in college by one of my chef-instructors. If the bones you have include a "knuckle" joint (actually the knee joint from the back leg), the bones will come apart when they are done. Cut the joint apart (if it's whole), exposing the cartiliage surfaces inside the joint, where the two bones articulate. As they cook, and the collagen in the cartliage renders in gelatine, the cartiliage will get soft, and softer, and eventually dissolve away. This exposes the hard bone under the cartiliage. As the stock continues to cook, this "curved" bone that supports the cartiliage will actually separate from the long bone of the leg along a "fixed joint" like the joints between the bones of a skull. At this point, the stock is considered done. I wish I had a picture of this. Making stock to this point usually takes me 36 hours. If using a pressure cooker might shorten that time.
If you run a search on my handle, cross-refereced with "Beef Stock 101", you should find a thread I started about a year ago, when I made a batch of beef stock. Hopefully the pictures are still being hosted.
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