Anyone know or ever had to replace the fluid in their trainer? I just wonder how long you can ride a trainer and have to replace the fluid???

May be much longer than I can comprehend.

The answer is: it depends.

First, what kind of trainer is it? The Kurt has a completely sealed fluid chamber that leaks very, very rarely. And if it does they will send you a replacement unit.

All the others have some kind of gasket system. The best of those seem to last a very long time, but it will depend on use as high intensity rides generate more heat which will be the main culprit in leaking seals. It sounds like the newer Cyclops fluid units have a decent lifespan and they too will replace failures on their current models and offer a new model at a discount if an older unit fails. I only have experience with those 2, but I'm going to guess you get what you pay for. If you intend to do frequent and high mileage or intensity rides you should invest in a higher end model. For occasional spinning a cheaper one will do.

As for actually replacing the fluid, I know a few on bikeforums have reported doing so, but even when they do it hardly seams worth it. If it leaked it means a gasket or seal needs replacing, so even if you manage to source the correct fluid you still may have more work to do. And as I mentioned, the better brands offer good warranties and on the cheap one you might as well just get a new one.