How long before i know this cymbalta (duloxetine) is working thoroughly?

Cymbalta (duloxetine): For depression, can take up to 4-6 weeks for full response at optimum dose.

Up to 6 weeks & more: A problem with antidepressants: there is no sure-fire way to pick one that will work for sure; and no sure fire way to get the correct dosage right off the bat. If you have taken Cymbalta (duloxetine) at a stable dosage for 6 weeks with no change in symptoms, you need either a changed dosage or another or a different medication; increasing dosage is usual next step before switching or adding medications.