Car when to repel – calculation

Hello forum, I have read on the internet more often now, in car forums that many buy a car, new or presenter and the after 2-3 years again to get again a new / presenter. I don’t get there at the moment, to what extent what you expect (except of course every 2 years to drive a new one) of it and wanted to ask, if someone is doing here like this or similar, and whether the model is worth for me (many drivers). r in March 2014 bought a Seat Leon TDI FR as a demonstrator (8600 km), which now stands, in September 2015 at 585xx km. So I drive pi times thumbs about 35000km a year, have 110km work journey every day. I think about it now, because the car is in a kilometer stage, where it would still bring decent money if I sell it. I do not have to sell it, because I am very satisfied, but wanted to explain to me how this “every 2-3 years or 50-60t km repel” Maybe someone is doing this and can give me some information about it.