This behavior is controlled by the Entry Screen the user is assigned to. Each user can have a unique Entry Screen, so you may have to first figure out what screen they're using. This is shown in the user modification page under Management -> Users -> Manage Users.
Once you know the name of the Screen, or you are creating a new Screen, go under Configuration -> Entry Screens and choose the appropriate type of screen for the data you're collecting.
On the right of this screen there are many different behaviors. The ones you're interested in are "Allow users to view past periods" and "Allow users to view future periods"
A check here allows the user to do that function. There is no limit to the amount of sheets in the past or the future they will be able to view, other than their start date.