@bryan this is a great question! I know individuals who have had the same paddle for years and love the feel of the paddle because they have come to know the unique parts of that paddle over the years. I believe it depends on the type of hits and how aggressive you are as a player (depends on if you are one to slam your paddle when frustrated 
).
For myself I tend to use the same paddle for around 5-7 months. I have had some paddles that only lasted 3-4 months because when trying to get a ball I end up slamming it on the ground by accident and then there is a soft spot on the paddle. I would bet that especially at the beginning your paddle should last you about a year or two before needing to replace it.
Now whether or not you replace it because you want a different paddle that is another story. Many people go through 3-7 paddles in their first year because they are trying out different paddles until they find one that they really like. Also, when you end up finding that you need more or less touch you may realize the paddle you have is a limitation and so it is a necessity to change your paddle.
This is a great question!!