It can take up to a year , I have had one tank that it never seemed to spred to back or the sides but covered my rocks ... As for the base rock I am not sure why coraline would not be spreading to them .. It does take awhile for it to be come premium rock tho .. I am in the middle of taking LR that has been on a shelf for nearly a year and has 0 coraline algea on it I will keep you posted on the progress . If possible you will want to get that test as that will give you a better Idea as to where you stand as coraline needs calcium to grow ...