As already stated a lot depends on how quickly you revover. I think you deff need to take at least a week off though some may go out for a loosening bike ride, walk or swim towards the end of that 1st week?
You can perhaps get back to light training the 2nd week and build up during the 3rd week.
You will feel back to normal a week after the Ironman, the thing you wont notice until you push your endurance is that you'll have underlying fatigue for 4-8 weeks. How quickly you get over that will depend on how conditioned your body is to endurance training and events.
With a 12 week gap id say you can get in some decent marathon training weeks 4-10 before you taper, aiming to re-peek around week 8/9.
Steteches and massage can also help the recovery cycle so dont forget that.
Having said all the above I know of a couple of people that have found that they re-peek 4/5 weeks after an Ironman with little more than just rest and light training.
Edited: 12/02/2010 at 16:48