Clinical studies show that a certain dosage of clenbuterol will be effective for 3-4 weeks. After that time, you will need to increase your dosage to get the same metabolism boosting effects.
Most users will start at a dosage of 20mcg per day and hold that dosage for 2-3 weeks. Every 2-3 weeks they will increase the dosage by 20mcg per day and hold it there for another 2-3 weeks until reaching a maximum dosage of 100mcg per day. I don't recommend using a dosage higher then 100mcg per day under any circumstance.