I'm finally getting close to finishing the clarified butter I've had in the fridge since I made it on October 31, 2005, when I did two pounds' worth of whole butter. I had it packed into a couple of glass jars; when I used up one, I opened the other. That's the bit I'm finishing now (having recently transferred it to a much smaller clean jar).
So you see, it can last a long time in the fridge. When you remove the milk solids and water, you take out the substances that are likely to turn rancid or help mildew to develop.
The important things to remember so that it keeps are:
- Always use a clean implement to take some out of the container.
- Don't let anything else fall into the container.
Both of these will keep the butter from getting contaminated with something else that can go bad, and then the mold or whatever attacks the butter.