The time it takes to make the pickled onions is already long. After the onions are picked, cleaned, chopped and brined, they are placed in a canning jar with brining solution. At that time, 10 to 20 minutes are required to process the jar in the pressure cooker, according to National Center for Home Food Preservation. The jar must sit for another 12 to 24 hours before the seal is checked and it is placed on the shelf to ferment.

The National Center for Home Food Preservation suggest that the jar of pickled onions stay sealed and on the shelf for as least three days for fermentation. Five days is even better. This time allows for the appropriate amount of time to achieve that tangy, sweet onion flavor. Three to five days is the earliest you should open the jar.

While the onions are sitting on the shelf, the brine and bacteria found on the onion's surface go to work to pickle the onion. The bacteria feeds on the brine, producing lactic acid. The acid grows in the jar, making the environment unsuitable to harmful microbes that normally cause food to spoil. This is what allows jars of pickled onions to sit on the shelf through the winter and spring until it is time to harvest more onions.

Before you crack open that jar after day three, inspect it. Throw away any jar that has come open in any way. This means cracks in the lid seal, or the jar's glass. Look for bulging lids, as this is a sign of contaminated pickled onions as well. You should also toss away any jar of onions that looks discolored.