Doing a Half Ironman before a Full Ironman

If you're talking about in the same season, like a 70.3 as a practice race leading up to a 140.6, a good window I read a lot is between 8-10 weeks before your 140.6 race. 

^^^This^^^ is a pretty good way to approach it.

Another way that I like for the first time IM athlete is to put a little more time between the two.  Anywhere from 12-16 weeks works well.  Then we train for the HIM, race it, take 1-2 weeks of unstructured goof off time, then resume training to bridge up from the HIM to the IM.  There are canned plans out there called bridge plans (I believe this site offers one).  Another good one is Gale Bernhardt's 13 weeks to a 13 hour Ironman plan.  

If you're an experienced triathlete that has done HIMs and IM(s) before, then I think 8-10 weeks can work (or less).  The fitness level, volume, and nutrition requirements are nothing new to you, so there isn't that much of a difference in rolling from a HIM to IM rather quickly.

If you've already done a season of HIMs and know how  your body responds to high volume training, and how your body responds after a HIM race, but are targeting your first IM, I think Don's advice of 12+ weeks might be a little more prudent.  It will give you more time to experiment with your nutrition and pacing specific to IM.  It will also give you more time to adapt to longer swims, rides, and runs that you may not have done previously in HIM training.

If you're talking about your first HIM and first IM, then I would actually recommend having a season or two in between.  If you come from a big endurance background then maybe not so much, but if the thought of your first HIM is to just survive...then you probably have a much longer road building the base for IM.  Of course that depends on your goals.  If one just wants to finish both events then it's certainly possible.  I didn't do my first IM until 2.5 years after my first HIM.

YMMV.

I think someone stated it above, but I'll highlight it again - understand how long it takes you to truly recover from a HIM effort (not just that your legs don't hurt anymore).  I know that for me I'm on the slow side of recovery, and if I ever do a full IM then I won't race a HIM in the same season, unless it's spaced about 16 weeks apart.  Another option is to race an Olympic hard about 8 weeks out - still a very good gauge of fitness, and recovery should be much quicker and not impact the overall training for the IM.