1996 Ford Ranger: Bought new April 1997. Sold December 1998.
1999 Dodge Dakota: Factory order, took delivery December 1998. Sold June 2001.
2000 E46 323i: Bought New September 2000, Traded September 2012.
2007 Lexus RX350: Bought Demo (never titled) February 2007. Probably keep it 10 years.
2011 E90 335d: Bought Demo (never titled) September 2012. Probably keep it 10-12 years.
So early on, it was 18 months or so. Now I've wised up, and am going 10-12 years. Could have kept the E46 longer--nothing wrong with it aside from some sagging c-pillar trim and 20k miles away from needing a clutch--but a deal on a never-titled 335d fell in my lap, so I switched out.