I'm going to cook Betty Wason's pot roast - https://food52.com/recipes/21975-betty-wason-s-basic-pot-roast - but am flummoxed by the cooking time.

It depends on what texture or level of doneness you are looking for. Typical pot roast cuts benefit from low temperature braising for long periods of time, which makes an otherwise tough bit of meat turn into a melt in the mouth, tasty roast. Usually beef hits medium doneness at 145F, and at that temp a pot roast cut will still be sliceable. It will be rather on the chewy side, but tasty. On the other hand, a roast braised until technically well done, 165F, should be falling apart tender. A bit also depends on the exact cut, as chuck may be ready sooner than a bottom or top round butt roast. The best way to determine when your roast is done is to check it, and remove it when it gets to the texture or temp you want. Technically it will be done at about the 3.5 hour point, and safe to eat. But if you can braise it for longer, it will be more tender and tasty.