The rate at which a battery discharges depends on the current being drawn from the battery. Current drawn depends on the electrical needs of components connected to the electrical system.
The rate at which a charging system can charge a battery depends on how much current the the charging system can produce. If you have an electric PTO clutch (just for example) drawing 3 Amp of current and the charging system is producing 3 Amp of current, the battery would neither gain, nor lose, any of its "charge" (not counting what was drawn by the starter to crank the engine). If the same PTO was drawing 3 Amp but the charging system was only producing 2 Amp, the battery would discharge given enough time. Knowing that charging system output increases with an increase in RPM, but decreases with a reduction in RPM, and that engines are designed to operate most efficiently at the "rated RPM", which is usually between 75% and 100% of the governed RPM, it would be a fair bet that most tractor engines will not produce enough current at idle RPM to keep the battery charged if there is any current higher than 2 Amp being drawn from the battery. But if there is no current being drawn from the battery, 2 Amp would be "Ample".
There is no simply answer, but to say that you have to put more into a battery than you take out or it will discharge comes pretty close. | 
