"Jose" wrote in message
...
So, how far over redline is it "ok" to operate an aircraft, and how
arbitrary is that red line (both on the tach and the airspeed indicator)
and how is it determined?

Check the engine manual. It will generally give detailed information about
what action(s) need(s) to be taken after exceeding some operational limit
for some amount of time.

For example (an invented example, not necessarily from any actual engine
manual), disasssembly and inspection of the engine if the RPM limit has been
exceeded for more than 5 minutes.

You'll find limits for RPM, temperature, manifold pressure, and possibly
some others.

Pete