* * *

Sure, they can be and are often longer, but not because you were forced to use long keys, it just happened that the thing you want to store in a hash table is a long string. The way you worded it made it sound to me like you were saying that one has to use long keys, not that in practice one often has to deal with long keys. But even then I am not convinced that this should give an additional factor in the complexity analyses, I think I would argue, at least in some situations, that calculating hashes of long keys should still be considered constant time, that for the longest keys. But I can also imagine to take this into account if the key length is not only big but also highly variable.