Mustard can be left out of the refrigerator and it will not go bad.
Mustard does not require refrigeration just like ketchup does not require refrigeration.
However if the mustard is the Dijon and horseradish-based mustard then those types of mustard must be refrigerated or they will go bad.
However regular mustard does not need or require refrigeration.
I've always just left regular mustard out on the table or in the pantry even after opening it and it has always remained good.
An opened bottle of mustard will last for up to 1 to 2 months in the pantry before it goes bad.
But if you refrigerate the mustard it does make the mustard last a bit longer.
When refrigerated the mustard will last up to a year but in my house we use the mustard well before a month and certainly before a year.