It depends on how reliable the car has been and how much I like it.
I have had a few in the 8 to 12 year range and a few in the 3 to 4 year range.
Right now I have owned my cars 9, 4, 3, 2 and 2 years. The one I have had 9 years I will probably replace next spring when it is 14 years old and I have owned it 10 years - don't want to spend my time chasing rust. The one I have had 4 years I would love to sell - worst 4 years of car ownership ever - but my wife won't let me
My E46 M3 I will keep for a long, long time and my F30 I bought new and will keep at least another 8 years - until it is 12 years old. My wifes new daily - who knows, depends and how she warms up to it.
Also, I have bought used and new equally for my daily drivers. Depends on the timing and spec of what I want and what is out there. My weekend cars have all been bought used.