From the research I've done, I understand that it can take anywhere from a couple of weeks to months, depending on the dosage, frequency, and the person.
There is no set dosage that applies to everyone. Your dosage is set by your doctor, depending on your own situation. It's, typically, based on your labs (Free T4, Free T3 and TSH) and symptoms.
Others on the forum have, usually, been prescribed 5 or 10 mg once or twice/day. Symptoms and labs must be watched carefully to make sure you don't swing the other way and go hypo.