Ouch. Maybe try setting it for smaller units and see how they go?
Also, when you say 3.5 day WU, are you talking about the 2 million step SMP units?
The system I'm folding on completes a normal pseudo-big SMP WU in 5 hours or less. But it's a waste of ressources because it could easily fold with the -bigadv setting. Then you will receive very large WUs (you will need at least 8 CPU cores to get one assigned and 6 GB of RAM are recommended) which have a very short deadline (4-6 days?) but you will earn enormous amount of points (I had up to 80k on a different machine in less than 3 days). I completed one on the current machine (was around 60k) before but I don't know why it isn't working anymore. I have completed two normal SMP WUs today and they were good, now I hope the bigadv WU will work too. At least I'm giving the machine another try 
That must be one heck of a system. I can't do -bigadv because the best system I have is an AMD quad with 3GB RAM and a 32-bit OS (WinXP SP3). It does a 2 million step WU in just over 48 hours. GPU unit output is about 1 unit every 24h if I let it run continuously.
I was looking into a 6-core when I built this system, but they were ridiculously expensive at the time, and I had no use for it. I passed on more RAM because I wanted a 32-bit OS for compatibility reasons (several programs I use don't work on 64-bit OSes.)
Hope your new WU goes well.
