For my PIC tutorials I use Molex 10 pin connectors on the boards, and join the boards using leads about 300mm long - these work perfectly fine with SPI - but I've never had any reason to try (or even make) longer ones.
I'd suggest you give it a try and see how it works - assuming you use pluggable leads, you could try a short one (to make sure everything works) and then try it with a longer one. I wouldn't have though 500mm would be a problem, as 300 is perfect. If you do start to have problems, you could always reduce the SPI clock frequency a bit.