It depends on the growth rate of the specimens, and possibly your cultural or mythical beliefs. Some foragers will tell you only to harvest during a full-moon or at sunrise the second day of the second month (I just made that up, but you get the idea) for spiritual or folklore-medicinal properties reasons. You can follow this or not, that's up to you. From a practical standpoint, harvest them when the caps are fully formed, dark red, and starting to harden or almost completely hardened. This is usually 2-3 months after they initially form.