When to replace old car

Im currently in a dilemma. I have a 2006 cobalt that is paid off. it currently has 100,500 miles. The only maintenance i currently have is oil changes very 3 months and an occasion payments of repairs around 500 dollars about once a year. I tell myself its not worth it to repair it any longer and i should just sell it but its not worth anything. For people who have had a car that is paid off and older when did u decide it was time for a new one?