The present Fermi tasks complete (under a good setup) is about 5 to 6 1/2h. So with a GTX480, you will complete your cache of tasks in about 11h. With Out-Of-Hours Internet restrictions in place, you will not be able to pick up any more tasks or report completed tasks, so your systems will sit idle for about 63-11h = 52h over 2 day weekends, and 7h every work day evening; 80h a week.

There are only 3 ways around this,
Increase the number of work units a Fermi can pick up (cache) - tech
Increase the length of the work units - tech
Keep an Internet connection open - you

If the techs could allow Fermis to pick up more tasks (10) that might be the easiest way forward. If the task length doubled, that would reduce your work outage to 41h (for a 2 day weekend), and normal work days would not be effected.

As for an Internet connection, there are many possible solutions.
You could leave the Internet on and increase OOH restrictions; perhaps limit connectivity to only the Fermi systems, possibly turn other systems off (no security issues), limit access to one port, use a key... possibly even use a second Internet connection out of hours. Of course all these possible solutions eat into IT management hours, never a good thing.