Oil Wager: three years
On the third anniversary of the wager, the numbers are in my favor: a barrel of light sweet crude costs noticeably less than the 19.2 Big Macs which matched it in value on December 1, 2005.

Here are the annual figures so far:
With this level of volatility, ten years is probably the very minimum term in which any kind of long-range trend will be visible. For perspective, here's the chart showing how far we have to go.

Sources:
http://www.economist.com/markets/bigmac/
http://www.eia.doe.gov/emeu/international/crude2.html
Here are the annual figures so far:
Oil | Big Mac | ratio | |
12/1/2005 | $58.75 | $3.06 | 19.20 |
12/1/2006 | 63.43 | 3.10 | 20.46 |
12/3/2007 | 89.31 | 3.41 | 26.19 |
12/1/2008 | 49.28 | 3.57 | 13.80 |
With this level of volatility, ten years is probably the very minimum term in which any kind of long-range trend will be visible. For perspective, here's the chart showing how far we have to go.
Sources:
http://www.economist.com/markets/bigmac/
http://www.eia.doe.gov/emeu/international/crude2.html