When should a ground rod be replaced?
Who knows, IMO almost never unless something unusual has happened to it.
We test electrodes with a three point fall of potential test like Mike (Trumpy) mentioned.
The cost of buying, renting, or hiring someone with one of these testers greatly exceeds the cost of installing a new rod.
So if you are not comfortable with a ground rod than I say forget testing it just add a new one to the grounding electrode system.
If you where to test it what value would you be comfortable with..5 ohms...25...100 ohms?
There is a ton of myth about what ground rods can do for premise wiring systems operating at less than 600 volts and what they can do for systems that operate at a relatively low 120 volts to ground.
Rods and other electrodes can do a lot for high voltage distribution systems.
If any one is interested I recommend taking a look at this
thread at Holt's. IMO there is a lot of interesting information.
That said never accept what you read on the Internet as fact without doing more research or accept that our current thoughts on grounding electrodes are entirely correct.
What does a grounding rod do for us at a typical home service?
now that the code requires 2 rods
That is an overstatement, the code does not require two rods, the code does not actually require any rods.
If the only electrodes available are rods one rod is fine if you can prove the magic 25 ohms.
If you have other electrodes you may not need rods at all.
[This message has been edited by iwire (edited 11-15-2006).]