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wrote:

I have a 7-year old A.O.Smith 40 gal water heater. Works fine, but I've
only drained it out once about 4 years ago. Assuming that that water
heaters last about 8-10 years, I have some questions

That's a generalization that doesn't necessarily hold up. Water heater life
depends on a number of factors, including how hot you run it, how much mineral
content there is in the water and how clean the water is.

- how often should I be draining and cleaning it out?

You don't usually need to drain it completely, but attaching a hose to teh drain
port and running the hose until the water is clear once a year is typical.

- does regular draining help prolong its life?

Yes. A major failure mode is mineral content building up at the bottom of the
tank. That makes the burner have to run longer to heat the water, if you have a
gas heater. Running the burner longer (or hotter) causes the tank to fail.
Electric heaters tend to fail from mineral buildup on the electrodes. Those can
be replaced.

Another common failure is the dip tube breaking. That allows cold water to
immediately exit the heater into the house instead at the bottom of the heater
where it can be heated first. People usually compensate by cranking up the temp
which casuse the tank to fail.

Electrode failure in an electric heater depends on if the top or the bottom
electrode fails. If the top one fails, you get very slow recovery time. If the
bottom one fails, you get slow recoveryand half the amount of hot water.

- how does one tell that it's ready to break (instead of finding out
when the basement is flooded with hot water)?

I've had heaters with pin hole leaks that lasted long enough for me to replace
them a few days later. If you see *any* condensation like symptoms on the floor,
time is running out.

Mineral build up symptoms are a crackling noise when the burner runs. Other than
the dip tube problem where the water gets cold too fast, I doubt there's any
other way to tell.