The largest growth will occur in the first 1-2 days and the rest will come through over the remainder of the 7 day period.
As an example, one of our founders recently tested out a giveaway and his account grew by 26,000 followers in 1 day and a total of 60,000 new followers by the end of the week.
Not 1 person questioned the validity of this and we haven’t had any feedback from customers with regards to any doubts over the rapid growth.
Ask yourself, when was the last time you visited someone’s homepage and took notice of their follower count from 1 day to the next?