It's been a while since I've grown lotus, but I'd say cut the pods while the pod is large and still green. If you wait too long, the stem will curl so that the pod is facing downward. As I remember, the pod swells as the seeds develop in circular depressions in the top of the pod, then the seeds pull away from the sides of the cavities they're growing in. Pick when the pod seems to be at its largest, and certainly as soon as you see any "daylight" between the seeds and the body of the pod. Hand the pods upside down to air-dry.

You might also ask this in the "Ponds and Acquatic Plants Forum." A more current lotus grower might have a different, and better, notion.