How long should I use mascara before I replace it?

It has been 8 months since I opened a new bottle of mascara. I haven't used it often, but the liquid seems to have dried up a bit. It feels like a waste to throw it away, and I think I could still use it, but is there a set expiration date? If it looks slightly dried out, should I switch to a different product? I think I saw somewhere that there is a way to salvage it, but I can't quite remember the details.