Note: Trust me, this is from experience.
Now as some of you know, usually mascara can only be used for 4-6 months as it is a breeding ground for bacteria. Since mascaras have a liquid consistency and you use it on your eyes then put it back in the tube, certain kinds of bacteria may be thriving in there. And if you find it's kinda smelling a little funky, you better toss it. It's not worth the risk.
But if you find that you still have plenty of mascara , hasn't hit the mark of 6 months and isn't smelling like feet but is getting a little clumpy here's what to do to extend it for a few more uses before you buy a new tube.