You have had the same mascara for 6 months and you still use it EVERY DAY?!
I use mine every day too, and buy a new one about once a month!
Oh - and for the smell thing that is true, some brands last longer than others (interestingly cheaper ones tend to last longer but thats cos they have more chemicals in them..) so DO go by the smell. Just once it smells a bit dodgy - get a new one and an old one and you'll notice the difference.
My friend used to be a make up artist, and she learnt this at college.