2000. No car, living in a big city while at school
Early 2002: Bought a junk yard Toyota for $100 (sold it on ebay a year or so later for $400

Late 2002: Bought a 2-year-old Mitsubishi for wife to use for work
2003: Sold the Toyota, not reliable any more, bought a 5-year-old 2-door Nissan for husband
2007: Kid#1 born, needed a 4-door car, sold the Nissan, bought 6 year old Saab
2010: Kid #2, need a second 4-door for flexibility, sold the Mitsubishi, bought a 2-year old Mazda
2015: Needed a larger car (widowed grandparent will live with us) with high towing capacity. Sold the Saab, bought a 4 year old ML350
So, with a bit more planning we could have done fewer replacements (e.g. gone with a 4-door upfront, but then we didn't even know if we wanted kids at that point), and of course we could have survived without a larger car now (Grandma will just have to fit in there between the car seats), or without the towing capacity (keep renting from UHaul), but each change felt logically justified. We can certainly afford it, even the ML which cost us twice what we had paid for any earlier car).
So I guess I am just curious - how often on average have you changed cars and how strictly you have followed the "drive it into the ground" rule?