Digital Performer 3 manages hard drive space by allowing each record-enabled track to allocate a certain amount of disk space - 2 Gigabytes, which amounts to over 6 hours of record time per track. By doing this, DP is able to start recording and playing faster than before, and use the hard drive more efficiently.
If you actually want to record 7 hours of continuous audio, DP would automatically allocate more space to that track. So that "Available" isn't the actual limit.

* Realistically, you'd be far better managing very long recordings, by separating long sections into either separate passes on a project, or (even better) creating separate projects for each song or section. Managing a 6 or 7 hour audio file would be quite daunting.