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According to most 'experts' cryo doesn't have to take a long time. Something on the order of an hour or two should be enough. But, from your experiment I would say that longer cryo in the first place would be a good idea. I always left mine in over night. A longer cryo might help but it can never hurt. You are definitely right that there is a limit to how much change can be made. Remember too that when various people say 'cryo' they aren't always talking about the same process. Dry ice is one thing, liquid nitrogen is quite another. Likewise, in the liquid is one thing and suspended above the liquid is another. It's hard to be sure you are comparing apples to apples. And then there is the steel itself. Even when we say 440C we may not be talking about the same exact alloy as that steel is made by a lot of different manufacturers.
In the end, I'd leave it over night - not likely to get a lot more out of it than that. And no matter what, once the cryo is done I'd follow with a temper cycle because if the cryo does make any change then it has also added stress and that stress should be relieved. The good news is that temper cycles are easy to do and even if an extra temper doesn't help it can't hurt so the smart money would just git 'er done...
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