At what point should you commit to ironman?

I agree with Matt.  You go into your first IM with the basic goal of finishing, unless you're an accomplished triathlete at other distances.  You never know what's going to happen during the race and until you've gone 10 hours +, nutrition is just a guess.  Rich Strauss from Endurance Nation gave a talk on strategy before IMLP and IMCDA this year.  At IMLP, he said, "those that don't plan on walking some of the IM marathon, have never done an IM marathon."  I think almost everyone walked at least the water stops.  I'm sure this advice was directed at first timers and that people with a lot of IM experience don't follow it but it made sense to me, a first timer.

Ernie

I used to think I wouldn't do a IM until I could swim and run a certain pace and was fit enough to not expect to walk the marathon.

I've changed my thinking 180 degrees by being part of BT for 4 years now and listening, reading and watching the training of many BTers. You can never train for every possibility you are out there just to long. Most folks do walk part of the Mary.

I doubt I'd be as fast as you estimate you might be...but I'll be competing as a 47 year old women. For me know it is about the challenge of training, preparing and getting to the starting line healthy to do my best on July 20th. I know I'll be doing some walking...I have found that I am faster when I do run/walk and the goal is to be as fast as possible.

The time estimate I read in Friel's TTB is good IM is 5x the time of your best Oly that puts me at 15 hours. Hey if I finish and I had fun and did my best given the circumstances of that day, I hope I'd be pleased that I was an Ironman.

Look at the times from any IM marathon.  I would say that very few people really run the run. 

Before you commit to an IM, I think you need to figure out what you want to get out of an IM and what are your goals in the sport.

One of the cool things about this sport is that there are as many different reasons why someone commits to an IM or any tri-related goal as there are athletes in this sport. And none of those reason are any better or worse that any other.

Once you figure out why you want to race or participate, then figure out if the commitment needed to meet that goal is worth it. If the answer is yes, then go for it.

Personally, I have held off on doing an IM race until I feel I can race for a Kona spot. I may never feel that I'm ready and therefore never race one, but that is life. Other things in this sport and in my life are more valuable and important to me than finishing an IM in under 17 hours. Many, if not most, others feel differently than me and I respect and admire that position as well.