I've got a ROM I build with calk's kitchen that is 6.5.x and after about ~1100 messages total (in all folders) the phone will start to lock up very frequently, manila will crash often, the phone will get the sleep of death for a few minutes, etc etc. It basically becomes immediately unusable. If I archive them all using myphone and sync, everything gets better again.

I get about 36 hours of battery life, and send maybe 30-50 messages a day.