Well, if the consideration is solely to minimise the balance to be paid, then the answer has to be ASAP. A new car takes its biggest hit in depreciation right at the start, from then on, the rate of depreciation will gradually slow. The drop in the first year will be the most significant, for each subsequent year the drop will reduce.
If however you want to consider the total cost of ownership over the longer term, then replacing a car with a new one each year is the most expensive model, even if you manage to get 0% finance.