USB-Blaster JTAG max cable length

The acceptable JTAG cable length depends on what you are trying to do with the JTAG. If you are downloading images to the FPGA with a human in attendance then the cost of an error is that you have to do it again - in this case a 99% success rate might be acceptable. If you have an automated system or are debugging a processor then any error will cause problems (JTAG does not have error correction or retransmits)

The chance of getting hit by a bit error also depends on how noisy your environment is - equipment like motors can generate spikes on the JTAG which will cause more problems.

If you are not happy with bit errors then I would recommend no more than 5cm of standard ribbon cable, or no more than 20cm of the flexible PCB which comes with the USB-Blaster.

You can extend the USB cable quite a lot more (USB has error detection and retransmits). I think you can chain upto 6 powered hubs if you need a longer cable. I would recommend extending the USB side of the connection and keeping the JTAG side as short as possible.