There is no definitive answer here, it all depends on how you use it. Keep it clean, in the appropriate working temperature range and above all moisture (water) free and it should serve the average knifemaker for a very long time. Good oils are designed for the heavy hard use that industry will give it with hundreds if not thousands of parts a day, a custom knifemaker that takes care of it will give it the easy life for certain. It is, after all, the consistent results over time that is perhaps the greatest advantage of a good formulated quench oil, so that is why taking care of it is so important, much the same way it is important not to abuse or trash a good Bridgeport mill. I can't stress enough for folks not to edge quench in a good oil, the quickest way to trash it is to catch it on fire. The thermodynamics of an edge quench are so far off from standard quenching that there really is no difference just to use a vegetable oil substitute, and burning up Wesson oil is a lot cheaper.
All I can say is monitor things, if the oil starts looking dirty or results start getting inconsistent then it may be time to change things out. One of the things that I really like about the Park Metallurgical oils is that they are clear, if you can't see the blade cooling clearly below the surface the oil is not at its best. Some common signs are staining, scaling or nasty deposits on the blade surface, if you knives are coming out of the quench not looking a nice, clean mottled gray your oil may be going south on you. I have been using the same batch of Parks #50 for at least five years now, granted I don't do much oil quenching but all the same it is still going strong. Once a year I empty the tank and clean all the salt flakes out of the bottom and put the oil back in.
