Lou has an anode rod usage chart in the FAQ section:
Anode facts and FAQs
If anyone tells you the anode rod is JUST a drain plug on a Suburban brand water heater, they need to educate themselves more.
Sacrificial anode rod protects your water heater tank from corrosion by attracting corrosive elements in water, so they attack the anode rod rather than your water heater tank interior.
Simply remove the anode rod from the tank and inspect it every three months. If it is mostly eaten away, replace it with a new one to extend the life of the tank.
To answer your original question, as others have stated you still have much life left in that one pictured. How long an anode rod last depends a lot on the content of the water in the tank, which is different in different parts of the country, municipalities, etc.
You had concerns about it being brittle near the bolt head. There is a metal rod that goes thru the entire length as you can see in the usage chart at 100% usage, so it's not going to break.
Here are some more pics so you can see this better: