where does a defective relay rest at?

You can't say for sure, because it depends on how it fails.

If the coil fails the relay will always rest at 87A based on the principle of how a relay works (the coil provides the magnetic field to pull the relay in).  If there is no magnetic field, there is no switch.

However, relays also have springs that can break, that would prevent them from returning reliably to the NC position.

Finally, the contacts can wear.  If you are switching a high load over an extended period of time, espcecially a load with a high in-rush current the contacts will arc each time they switch.  This arcing will eventually cause the contacts to quit connecting.  It can also weld the contacts in a particular state (the high current state).  This occurs when the contacts spark and the spark mechanically bonds the contactor to the output pin, and the bond is stronger then the spring.  Usually, a swift blow to the relay will free the contacts and you can get a few more uses out of it before it fails all together.

In my experiance, the contacts fail 99 out of 100 times when a relay fails.  Because of this, it is simply impossible to predict the state the relay will fail in.

Anywhere from 100,000 to 10,000,000 cycles I would guess, based on the quality of the relay.