I've been know to keep my workstation up and running for at most a week before rebooting.
I find it necessary because I notice a significant slow down in the performance of my workstation after that period of time.
I haven't been able to figure out why until a few weeks ago.
My workstation has 2GB of internal memory and as per the rule of thumb, I have a 4GB swap partition allocated on a 250GB hard disk drive.
I really can't claim to know what's happening in the background but my hunch is that the stuff that the virtual machines and applications I keep open that are initially on internal memory are gradually being moved to the swap partition.
Swap is generally slower because it's on a disk drive.
I found this article from FOSSWire that suggests turning off swap to speed up your system.
Its working well for me so far :-)