According to the ATA specification 18" (45 cm). If they are very long, you risk data corruption. However, some are using 24" (60 cm) cables without problems, so I guess it depends on the controller and drive whether or not this works. Further, long cables are more likely to cause problems if you also have two drives connected to it.
<i><b>Engineering is the fine art of making what you want from things you can get</b></i>
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