There are a multitude of designs and some are better than others. Decently-designed ones will last for years. They don't sink with time - they do degrade around the plastic or silicon runners and seals. I feel the same as the above post - you can probably delay the inevitable replacement, but you will always by fighting to stop it leaking. If you are replacing it, look at a couple of the top-quality designs to see how they work and how easy they are to clean. Even if such designs are not in your budget, you can look for features on cheaper designs that are similar. Amongst better designs are Matki and Daryl.