Wedding dress worries!?

i get married in 53 days &i have my dress fitting at the end of the month, iv ordered th ellis 11003 in size 16 the problem is im an 18 , i tried the 16 on in store and it was a bit tight so saleswoman recommended an 18 but i was stubborn and ordered the 16...iv since lost 7lb and am continuing with the diet and exersize but im wondering approximatley you have to lose to drop a dress size? i no every one is different but its jus a ball park figure im after?