Ive had mine for a while now.  I always try and squeeze the air out of the bottle then put the cap back on.  The moisture in the air may cause it to start to set up.  As long as its a nice golden brown color and not milky looking, it should be fine.  I worked in a plant that made tiles out of recycled tires, and we used this stuff to make the tiles.  If the tote of glue was milky looking, the tiles never turned out good.  Also, dont let it freeze.