How long is synthetic oil good for?

Thanks everyone for your replies. The reason for the low mileage is that this is a 1969 Impala that I’ve restored (Rebuilt engine, trans and so on). When I do drive it though, I make a real effort to drive at least a good 10 to 15 miles. I’ve also gotten into the habit of not starting it at all during the winter, when just sits in the garage, because I’ve heard this does more damage than good to the engine.