It depends on the rate of fluid loss.  A minor or slow leak will allow you to drive as long as you maintain the level in the normal range.  You will have to establish the rate of loss and replenish as necessary.  It should be obvious that if fluid is running out as a stream, that you won't go very far.  A transmission will usually operate "normal" until the fluid loss is a quart or more.  Then the unit will exhibit abnormal operating symptoms and internal damage is occurring.  What started as only a leak can result in a major repair bill if ignored!