In alt.home.repair, on Wed, 5 Aug 2015 18:42:25 -0700, "taxed and spent"
wrote:

Are they supposed to be changed preemptively after so many years, but before
they fail?

No, they last 10 or 20 years or more and there's no reason to replace
them early.

It's also not true that the water heater will always have a lot of crud
in it. I mistakenly replaced mine after about 10 years, and I cut it
open and there was just about 2 tablespoonsful of crud. It would have
taken more than 300 years to reach the element.

What you should know is that when the time comes, you don't have to
drain the tank to replace even the lower element. Turn OFF the
electricity to the WH. Turn OFF the water going to the WH, turn ON the
hot water somewhere until no more water comes out. Then unbolt or
unscrew the element, pull it out and lickety-split put the new one back
in and start screwing it (or the screws) in. When I did that I spilled
only about a tablespoon of water into insulation just inside the outer
cover. It evaporated eventually.

Saved all the hot water, and a bunch of time that it takes to drain the
tank and refill it. .