Every herb is unique and dosages vary accordingly, but there are some general guidelines that can be very helpful.
Extract dosages can be measured in drops, dropperfuls, milliliters (mL), or teaspoons.
A dropperful is about 15-20 drops, or 1 mL.
A teaspoon is roughly equivalent to 5 mL.
The most commonly used dose for tinctures is 20-40 drops or 1-2 dropperfuls. In general, the more acute a condition, the more frequent the doses. Safe dosage ranges are fairly broad with most, but not all, herbs.
We highly recommend that you use a good herbal reference and become familiar with the individual characteristics of the herbs you are using.