Hello Wonjin Han,
For A to B type cable, it is 3 m for USB 3.0, and 5 m for USB 2.0.
For compliance tests with B connector fixture, it is a requirement to use 3 m cable.
For the type-C fixture, the maximum cable length is 1.8 meters when testing.
The above cable lengths have been used while testing the cypress USB products (SD3 as well) and that too, with a USB-IF certified cable. If a non-standard cable or a cable with greater length than specified (compared to USB spec), the results may differ as the signal quality is affected.
Other than the above-mentioned cable length (which are recommended by the USB spec), SD3 hasn't been tested further, because the test results will be highly subjective to the application.
If the customer needs to use higher lengths of cable, it is recommended to either use a Retimer or a Hub in between the device and host to maintain signal integrity.
Regards,
Yashwant
