Top it off anytime you wish.. Li ion batteries don't take a set. The Apple device works with the Apple charger to regulate current draw. Above 80% it's reduced, and at 100% it will stop and start, keeping device between 99% and 100%.
Li ion battery capacity will drop to about 80% of total new capacity after approximately 500 recharges of 100%. That is going from zero to 100% equals one recharge cycle. The effect is cumulative, so five 20% recharges equals one 100% recharge. Simple thing for you is not to worry about recharging. Just plug it in, the Apple engineers have made it simple for you. The once every month or two to let drain down and recharge completely is to recalibrate the percentage meter only, giving you more accurate readings.