Mine has 18 months of life, apparently. I wouldn't leave it that long, though. For me, the best way to know Emrys is getting the UV he needs is to buy in September, just before winter starts setting in, so it's at its newest over the winter months when he'll need it the most. In summer, it will just be supplemental to sunlight so the UV isn't as important. Then, it'll get replaced again in September for the following winter. They say they have lives of far longer than a year, but I know they start wearing down straight away and I don't have a UV monitor so I like being sure I'm not just shining useless light on him.
Average life depends on brand. T-Rex bulbs can last 18 months or more. ZooMed is commonly 12 months. Exoterra can be as little as 6, and up to 12. What kind of MVB do you have?