If you notice that you're down to the last 10% of a tube of toothpaste (or bottle of shampoo/dishwashing liquid/box of flea powder/whathaveyou) and buy a new one, then using the last 10% will take as long as using the first 90% did. (Of course, if you don't notice, the last 10% will take one day to use up.)

This tube of toothpaste has looked like this for about a month now, I think, and it just keeps producing toothpaste.