DMSO can have a relatively long shelf-life if kept in ideal conditions. We always recommend siding with caution whenever in-doubt.
Although many will say that DMSO can last "forever," it is absolutely true that it can degrade over time. When this happens, there are changes in it's efficacy and potential side effects.
We recommend using your bottle within 2 years' time.
Caution: If you are unsure about how you stored your DMSO, or you question it's age and freshness, it is always best to carefully dispose of your bottle and purchase a new one.
If you notice a change in odor, or you notice a visibly cloudy appearance to the liquid when at room temperature, it maybe have degraded and should not be used.