Well it depends on a lot of factors. What kind of condition is the vehicle in? How much does it get driven, whats the relative cost of ownership vs a new one with payments, was it bought new etc?
For me its either when I get bored, it starts costing more than a new one, or when it stops being reliable. A new Explorer Limited costs what, $38,000? Thats a payment of at least $500 a month depending on whatever the average finance term is nowadays. A lot of factors go into it. I would have kept my old 98 Lexus with 160,000 miles on it. It started to cost some for repairs, struts, ball joints etc but nothing unforseen, it still looked good and drove great, much cheaper to keep the old one than pay the $1000 a month payment on a new one and I was still happy with it, but my company does my car and they were getting uncomfortable paying for the repairs. Everyone has a different "time".