This is another popular argument against Medjugorje. One must understand that the visionaries grew up in a very poor and sheltered communist governed village of 95 families. They knew nothing of apparitions.
They had never heard of La Sallette, Lourdes, Fatima, etc. When the apparitions began visitors began speaking to the visionaries about these other apparitions. From what they heard from these visitors they assumed that Our Lady would appear for the same short length of time. This was merely an assumption and proved incorrect. The important point here is that Our Lady did not say she would only appear for a short time.