Ye olde wiring harness? When to replace?

How many of you guys are running the original wiring harness in your cars?

Mine is fully intact and aside from a non functioning fuel gauge (probably the sending unit as others seem to experience) everything worked last I tried. That said, some of the wiring is looking brittle, as you might expect from a 41 year old vehicle.

How do you know when to replace the harness? I assume you shouldn't wait for a failure (as this might come in the form of a fire...no?)

Is it as big/frustrating job as it seems it might be?

Thanks for any insight!
-Mike