Maximum length of SWD signal lines?
No fixed length.  Link goes to design info for SWD.  Download the PDF and check section 2.12 on "JTAG signal integrity and maximum cable lengths."

Seems to depend on the device, the cable, and the clock speed as to what length you can achieve.  

It appears that it is expected that the drivers are low power (mention of less than 4mA of drive current.)  You can expect trouble as the capacitance of the line increases with length.  Longer connection=higher capacitance= more problems.

The documentation also suggests that lowering the clock speed is not hard, and that adaptive clock speed would be a way to deal with long cables.

The documentation provides some more suggestions as well.