In practice that is not how it is going to go. The military disagrees also, and while they are not oracles about how combat goes, they do usually have a good idea of the timeframe of conflict. If anything, they'd tend to underestimate the length of conflicts, and they seem to have arrived on the 3-6 month figure.

At the outer edge of that, all prewar stocks are essentially fired off and you're relying on your industrial base to replenish.