The dogs for our Whirlpool DD machine lasted about 10 years before they needed replacing. I think the reason they needed replacing is that Karen used to grab the agitator by the top and swish it back and forth a few times to mix the detergent in the water before adding the clothing.
This was the problem which led me to this website. We called a few repair places and some said to just "junk" the washer and buy a new one, others said the job would be around $300.00. Based on the advice offered here, I bought a FSP Agitator Overhaul kit for about $25.00 and did the job myself in about 20 minutes. Replaced the dogs and the seal. Easy.
They are still holding up. But we don't use the top of the agitator to swish the water around anymore.