A well-cared for slime can last months or even years. Slime doesn't necessarily go bad, however it does shrink over time. It also tends to melt down to its natural glue state when left untouched for a while - this can easily be fixed by adding a few drops of activator. Slime will likely need to be touched up at least a couple of times a month to keep it in good standing (you can choose to do this whenever you notice it is starting to melt down).