My suggestion would be to keep it on a per-bookmark level, but also allow setting a default expiry time on a per-subfolder basis.
Imagine you have your 'wormholes' subfolder, you can right click on it in the places window or whatever and set this folder to have a default expiry time of 48 hours. When you create the bookmark, in the window that pops up you have the dropdown to select which subfolder you want to save it in, and then you have the radio button to select the expiry time. When you pick your 'wormholes' subfolder from the dropdown, it then automatically sets the radio button to the '48 hour' setting. If that's what you want, then you don't need to do anything else, but say you see this wormhole is EoL, so for this particular bookmark you then manually set the radio button to 4 hours instead. This particular bookmark will then expire in 4 hours, but doing that doesn't affect the default setting for the subfolder so the next bookmark you create pops up with the wormholes subfolder selected and the radio button already set to 48 hours again. And if this next bookmark you want to create is, say, a gate tac rather than a wormhole, then you pick your 'tacs' subfolder and the radio button automatically flips to the 'never' expiry that you had previously set as default for that subfolder. ('Never' would presumably be the default default expiry setting for subfolders that you haven't specifically set something else as default for.)
This seems like the best of both worlds to me - you still can have granular control on a per-bookmark basis of exactly how long you want it to last, but in the cases where you don't need that level of control you have the convenience of subfolder-level default durations that you don't need to mess with.