Used once daily, full sublingual dropper, how long wil 15 ml bottle last?

Hi there! If you fill that dropper to the 1ml mark, then it should last for 15 days. You can also count drops: 20 drops = 1ml. So, again, 20 drops per day should last 15 days. However, I would recommend that you take 10 drops twice per day, rather than 20 drops once daily. I hope this helps!