Poly water tanks can last a very long time and the poly water tank I have that I use to water cows with is 15 years old right now and has provided lots of water to the cows.
If I needed to replace it today it would be perfectly okay but I'm sure it will last another 10 to 15 years just fine without needing to replace it.
Some people have reported that they have some poly water tanks that are 35 years old which is a pretty good or more great lifespan.
Poly tanks will usually outlast a steel water tank because they cannot rust like the steel tanks do.
So you should get at least 20 years out of a good poly water tank and it may last even longer than that.