For the past 10 years, around every year, or 10,000 miles. Prior to that, I trusted garages to rip me off until I lost patience with them, signed up here, and started doing everything myself.I wonder how often you have changed the ATF in all those years?
First ATF change I did around 2008 looked like chocolate milkshake (I have a pic somewhere!), but since then they've been reasonable due to low mileage since it's not my daily driver any more. Pic I posted yesterday was the first time in 10 years I've changed the fluid and instantly thought it doesn't look right.
Car was driven by a relative for a bit last year, but covered less than 5,000 miles.
Edit: My mistake, found pictures from some years ago - was the front diff oil which looked like chocolate milkshake first time I started servicing myself, not the ATF.