They'll be fine, modern batteries and Apple in particular go to lengths to ensure this, are designed to be topped up whenever. There's no memory effect like there used to be and the longevity of the cell itself won't be affected.
You'll often hear mention of draining batteries completely every month or so to keep them in good health and properly calibrated. While there's nothing wrong with that at all and if you can do it once in a while then absolutely go for it. But Apples (and many manufacturers) battery calibration is accumulative. By which I mean the circuitry tracks very carefully each partial charge percentage and when each of those little percentages adds up to 100% it's counted as one charge cycle, that's how it keeps itself in calibration which is important to its overall health.
The AirPods are very specifically designed to handle being frequently topped-off, so you've nothing whatsoever to worry about from using them little and often (or otherwise for that matter.) The batteries have a specific shelf life and short of a fault, they'll be fine for hundreds and hundreds of complete charge cycles before they start to degrade. Even when they do all that will happen is their running time will slowly but surely get shorter, which is slightly less of an issue with something you'll be charging as frequently as the AirPods.
