You should get your property re valued in any of the following circumstances:
You’re interested in selling and it’s been a while since the last appraisal.
There was a major change to your property (e.g. burned down)
You think there has been an upward or downward shift in neighbourhood pricing (you can usually tell by how homes like yours are selling or what they’re asking for)
The mortgage company is asking for one, but you haven’t had one done recently enough to satisfy their requirements; they may be able to give you a sense of what property values have gone up or down since things were evaluated last time. It’s worth recognizing that this may not always mean that values have actually changed.