If your research runs over 7 days, there are three ways you can manage your availability using the existing calendar functionality:
1. Set black out dates
Black out dates can be set in the "More availability options' portion of your calendar settings
2. Create multiple calendars
If you would like to manage your invites, you can create multiple calendars with different availability. i.e. Week One Cal, Week Two Cal etc. Then, when sending invites you can select the appropriate calendar that matches the time period you wish to interview them.
3. Update your calendar at the end of each week
You calendar availability updates in real time. So, what you can do is set your availability for the first week (by selecting 'from' and 'to' dates that span a week), then send your first round of invites.
If at the end of the first week you have not yet reached your target number of interviews, you can edit your calendar to reflect the coming weeks availability. You can do this by changing the 'from' and 'to' dates, and then sending a new round of invites, or 're-inviting' those that are yet to confirm their attendance. Newly invited (or re-invited) respondents will only see the newly set availability.