Completely dried up I would just toss it... one supplier has the 3.4oz for $25 bucks now and free shipping over $50. I bought 2 in the past (not at that price) and it was ~7 months before I opened the second bottle and it worked fine... just finishing it up. The key seems to be getting it completely sealed up when finished, keeping the air out. I clean the threads with 99% every so often to make sure it can screw on tight. Also I keep mine at a steady temp in my lab.

On another board someone mentioned putting a tiny amount of alcohol or witch hazel in the bottle if it was just starting to harden up. I never tried it though.