HOw early do you plan a HIM or IM?

If you are looking at some of the M-dot branded races, then you might need to plan on registering as soon as it opens. 

Otherwise, I usually have an idea of the races I want to do and then sign up for them about 5 minutes before the registration fees go up.  Although my training plan for my last HIM was 27 weeks, so I guess you could say I started training almost 7 months out.

If you want to do an "official" IM event in the US you will have to sign up a year in advance.  More of the overseas ones have flexibility in signing up, but then you have to factor in more for travel of course.  For the non-WTC or NAS sanctioned events (the irondistance but not branded races) you can usually sign up as close as a month or two in advance.  The distance is the distance, so do whichever suits your timeframe and budget.

For a HIM, it all depends on the one you want to do.  The one I do every year is in June and opens for registration usually in August the year before.  The last two years it has sold out in about 2 weeks.

Depends...

On my HIM, I mulled it over for 2-3 months and finally registered 7 months ahead of time.  I booked a hotel before I registered for the race, because I figured that would be the limiter. 

On my IM, I mulled it over for close to a year and signed up a year in advance.  NAS races (the official M-dot races in North America) usually sell out a year in advance, and many people volunteer one year to sign up for next year's race. 

Some people commit to their IM goal 2-3 years in advance, well before you can register for the race.  It takes time to build the swimming/cycling/running base and rearrange your life-priorities in order to train for an Ironman. 

Most M-dot races sell out quickly, as well as some of the more popular off-brand races.  So you need to make the financial commitment pretty early - at least a year.

A local long course race that I'm planning to do is still open.  I actually put a month of training in before finally paying the entry fee (about 3-4 months before the race).  However, I mulled over doing it for about 4 months prior to dropping the cash.

I am planning on racing the Augusta (GA) 70.3 next September and noticed it's already 20% full (or 80% empty if you're a pessimist). There's still over a year left and ppl have already signed up...in my opinion, I have to sign up this year or be left out in the cold...

one year out, i say!