Just using extension cables? About 0 meters.. Unless you open up the HMD (and void your warranty). The connections on the PCB in the pimax are normal ports however.
You have to get an HMDI repeater (a 4K capable one for the Pimax) and put that between the original and the extension cable. For the USB part, get a USB 3.0 extension cable to get enough power through.
There's already an older thread on the forums herewith different types of solutions for extending the cables.