Should I manually turn off my aluminum keyboard and magic mouse when my iMac sleeps, or is there an automatic low-power mode?
( BONUS: How often should the batteries be replaced? )
The mouse and keyboard sleep automatically. I've only turned mine off when away for extend periods (e.g. vacation.) I've seen 5+ months battery life (with fairly light usage) this way. If it were my primary machine I suspect ~3 months would be good battery life.
As of today my Christmas-present wireless keyboard reports 17% battery remaining on its first set of alkaline batteries. That extrapolates to 0% at 105 days; it obviously won't run to 0% so reduce that to somewhere between 87 days (today) and 105.
I am not using any other bluetooth devices; there was a battery life issue - for which Apple releaed a wireless-keyboard firmware update - when the wireless keyboard was used with other bluetooth devices. See:
Answer to your bonus question: For me, not until they die. That looks like about 3 months+ of run time for my usage pattern, the updated firmware, and no BT mouse, based on a sample size of 1...