22 feet on an A-body, so maybe 24 ft for a B; That's from battery switch near R/R corner of car, up through existing drain-hole in trunk, along rt side crossing over behind rear seat bulkhead, running inside interior rocker trough, up through firewall to starter or power post. There's also the alternator-kill wire to plan for if you run it that way. Then there's the cable from battery (also @ R/R) to switch, which I usually buy pre-made, as well as the grounds battery/frame & engine/frame.
Depending on starter access, you might want to run essentially a pigtail from starter to a plug for the starter wiring.