Buying glue by the gallon costs less, but it may not be a good investment if you use glue infrequently. After a while, leftover glue can develop a bad smell. According to the folks at Franklin Adhesives, your best
bet is to toss the old stuff and buy a fresh bottle.
Glues have a preservative in them to keep bacteria or fungus from setting up shop. But the
preservative only delays the inevitable. Your smelly glue may be
OK; a bad odor does not necessarily mean the glue has gone bad. But
consider what your project is worth and whether you want to risk joint
failure for the price of a bottle of glue. Most woodworking glue has a one-year
shelf life. Even though it's tempting to buy a big bottle to
save money, it's better to buy glue in a size you
know you'll use up in a year. Here's a good tip: Write the purchase date on the glue bottle. That way, you'll know when it's time to buy fresh.