Since 2001, the US Dollar has lost half of its value every 4 years. Of course, its actual value dithers about, sometimes more than this theoretical value and sometimes less, but as you can see from the chart below, it tracks very close to this decay line. The chart below also projects this line into the future, giving a reasonable guess at what the continuation of the last decade's policies will do to the USD's value in gold terms.
Let me be clear: there is nothing "natural" about this decay curve. It is the result of carefully crafted monetary policy. Central banks wield huge power over the value of their currencies. This is particularly true of the Fed and the USD.
Will these policies continue? There is no way to be certain; if debasement is pushed too hard, people may lose confidence, leading to a dollar collapse. But to reverse the trend would push interest rates sky high, and result in budgetary and monetary chaos as well. Thus a continued steady devaluation of the dollar is likely the "best case" central bankers can achieve.
When the USD moves much above or below the forecast value, there is a tendency to revert to the predicted value. As the chart below shows, deviations more than 10% seldom last very long. These extremes may be tradable if you are willing to wait a year or two before taking profits.