I use all my Manyu on a daily basis, right after clearing all the daily missions that might yield them (whether as a reward, or picked up along the way, like the gacha daily). That also means I always upgrade at least one girl of each type on a daily basis, which is pretty convenient for the upgrading daily. So my mailbox is emptied at least once a day, and I tend to have some room to spare in my harem. Just the current overstock of birds and dragons (10+ of each, so well over 120 dragons total) is getting a bit problematic, but sooner or later a few girls will be ready to evolve again.