The access period starts from the date your first friend joins.
Example: if you invited your first friend and he opens an account on 15.11 and the second friend you invited opens an account on 14.12, you will get 16 weeks access, from 15.11 until 14.03.
If you have a break between accesses to Super Deals, the new access period will start from the date in which your next friend opens his account after the break.
Example: If you had 16 weeks access to Super Deals, but then you didn't invite any other friend, you have a break of 8 weeks. If you then invite a new friend and this friend opens an account on 15.05, your access period will start from this exact date.