Depending on the location and the depth of injection, Juvederm usually lasts 6-9 months. Since the skin around the mouth is very active (talking, eating, etc), injections every 6 months is reasonable and safe. The skin around the mouth can require a relatively large amount of product compared to other areas on the face, which may explain your suboptimal results. Placing some filler in the cheeks can help "lift" the skin around the mouth and give more visible results.
Juvederm Ultra/Ultra Plus lasts anywhere from 6 to 12 months. You may notice that it does not "last" in the nasolabial folds because putting it into the folds doesn't address the main process of aging. Nasolabial folds form as the result of atrophy of the malar (cheek) fat pad and recession and downward tilt of the maxilla (bones in the cheek areas). Revolumizing this fat pad and cheekbone area with either Juvederm Voluma ( a new Hyaluronic acid filler that lasts 2 years or more and FDA approved for just this purpose), or one of the collagen stiumalating fillers (Radiesse or Sculptra Aesthetic) will achieve a "lift" of the midface, diminishing your nasolabial folds and giving you a younger and fresher appearance.
Hope this was helpful and good luck!
If you had full correction of your smile lines on both occasions, the Juvederm will probably last about one year at this point. If you did not have enough at each session, you will not be happy with the result and it will not last as long. Speak to your injector about your goals.
Juvederm can last 6-12 months depending upon the form of Juvederm you used. In addition, I fill to achieve a result that the patient is satisfied with. This could be a 1/2 syringe, 1 syringe or 2-3 or even 4-6 syringes. Everyone's needs are different.