With proper care and if you avoid puncturing the waterbed mattress they can last a long time and sometimes longer than a regular mattress.
A good waterbed mattress should easily last 15 to 25 years.
My mom had one for 30 years before she got tired of sleeping on it and decided she just wanted to buy a new regular mattress.
She said that the waterbed mattress used to be comfortable but as she got older it started hurting her back more and it was getting harder and harder for her to get out of the bed because it sloshed around.
That waterbed mattress was still okay but she threw it away and got a nice new bedroom set which was a good thing after 30 years of the same bedroom set.
But if you get 10 to 15 years out of a waterbed mattress that is a good lifespan for them.
I usually want to get a new bedroom set myself after 10 years so it doesn't matter to me if the mattress whether waterbed mattress or not lasts longer than 10 years.