I've had BI running for upwards of 6 hours per session and not noticed any slowdowns so far. Running on an Athlon 2800 with 1 Gb RAM and an ATI 9800 Pro with 25Gb spare HD capacity. I've not upped any default windows memory swap/virtual memory settings since buying this PC 2 years ago.

Could it be linked to the number of battles per session or something - I've not been too battle intensive during most of my prolonged sessions? Or maybe I'm just lucky if this is happening to loads of other folk...