Scott's answer is good. And I assume you're talking about actual depreciation--actual value--and not taxes. If you're talking about it from a tax standpoint, see an accountant.
As a practical matter, most mobile homes have a lifespan of about 30 years. Some won't last as long. I've seen others last longer. So, on average, about 3%-3.5% a year, though it's not a straight line. Just like a car, there will be more depreciation at the beginning. It's not quite as bad as a car, but the depreciation is a bit faster in the first few years.
Hope that helps.