I was going to say when something non-service gives up the ghost. But then I love the Ti, and as there's only about 350 - 400 of them left on the road I keep fingers crossed resale prices will rise in a few years.
Having recently changed jobs and being home based the Ti doesn't get out much and the missus nags me to sell it. She's also well cross about the oil she's dropped on the new block pave drive. However, we've spent a bit on top of the ÂŁ350 I bought it for (3 new tyres, belt change, new rad, new alty, new water pump, rear seatbelts, approx ÂŁ1500 all in). Because of that monetary input she thinks we should maybe keep it cos of the money we'd loose if sold.
Also it's a brilliant car in the snow.
As Plezier says, I think it depends how much you like/love your car.