This much we know: when Dean turns up in the Impala in the pilot, Sam doesn't question it, and when Constance possesses it, Sam asks "who's driving your car". We know that they hadn't spoken for two years prior to Dean's arrival, so we can infer that Dean's had the car at least that long, but since it's second nature for Sam to think of the Impala as Dean's car at this point, it's likely that he'd had it for a good long time before Sam left for Stanford.


(Although, realistically, it's unlikely Sam would refer to the Impala that way if it had ever been the family car. He'd just call it 'the car'. IMO :P)