This is quite a new idiom, coined after wide use of this when describing the mileage – which is the distance a car can go on a full tank of gasoline. Because no test can precisely simulate all driving habits, the numbers are usually accompanied with “Your Mileage May Vary”, or YMMV for short.
You can, however, use it to relativize almost anything. Let’s say you’ve been to a restaurant that makes, in your own opinion, the best burger in town. But, because you know this is subjective, at the end you might add: Well, it was the best dinner I had this month. But your mileage may vary.