How fast does one need to swim 100's in order to simulate a goal IM swim?

The basic question asked would solicit several other questions: How many 100's? 
How much rest? 
What kind of training are you doing? 
What kind of shape are you in? 
How old are you?

You also need to keep this in mind: "It's not what you have, it's what you do with it". To make the most of pace training, you need to determine the pace and work with and around it. A 1:12 per 100 pace can feel real good at the beginning and like death at the end.

OK, first some disclaimers and then we'll do the math.

The X factors: Age and Taper.

Maybe I should start with the last question. Age has a lot to do with recovery. Hard training can really wear an older swimmer down. Most of us have noticed an increase in recovery time between about the ages of 42 and 47. All other things being equal, an older swimmer can often surpass expectations with a taper.

Also, someone who has been training very hard can also get a major boost with a taper. Conversely, someone who has been doing only moderate work will not get much from a taper.

Open Water Disclaimer:
Did you ever hear this one:"I swim open water faster than in a pool because I don?t have to turn."

BULLSHIT.

You might swim open water faster because you have a wetsuit on or more likely the swim is measured short. 90% of all open water swims are measured short. Even the worst turns are faster than swimming. in the open water The standard adjustment is one second per turn. Example: 200 meters in a 25-meter pool has 7 turns, in a long course pool there are only 3. Swimmers average 4 seconds slower in a long course pool.

Last but not least you can not discount the lane lines and the black line. Except for "cable swims, like Ironman Lake Placid" in is nearly impossible to swim a straight line in open water. (Cable swims are where they run a cable the length of the course and you can follow the line without lifting your head.)

Pool Swimming pace and interval
Most coaches I know use the T30 (30-minute swim), but I (Frank) prefers the 1-hour swim. The average pace for the one-hour swim becomes the OD Pace (over distance) + 10 seconds becomes the OD Interval (usually rounded to nearest 5 seconds). This is the base training interval. Depending on the swimmers distance swimming ability, their 1 mile (1650/1500) pace is 3 to 6 seconds faster than OD pace. We don't use this calculation to determine the one mile pace, just how to begin training for it.

Here are my two favorite quotes when I talk about pacing:
"If you can't hold 1:20 per 100 meters you can't break 20:00 for the 1500."
"In a long distance swim, how fast you swim your slowest 100 is more important than how fast you swim your fastest 100."

(I saw that later quote a few years ago in Swim magazine and got pissed off that someone stole it from me. My wife then pointed out that in small print under the quote it said "Frank McQuiggan, EP Masters." To this day, I don't know who submitted it.)

I will now use my own training to explain how I set it up. Keep in mind that my goal is based on an all out swim with a taper and a good psyche up.
20:00 for the 1650 yards or 1500 meters was one of my goals for a long time. That is 1:20 per 100 meters and 1:12.7 for yards. At this point my one-hour swim pace was 1:17 so I used a 1:30 base interval in a yards pool. After building my endurance base up for the hour swim in January, I would do sets like this using a goal of 1:12 to make it easy. Not only is it a round number, but I get to anticipate the push-off.

20 x 100 on 1:30 descending from OD (1:17) to 1650 pace by # 15. This works goal pace at "crunch time" until then end.

In better shape I would switch this to holding goal pace (1:12) for the 1st 15 and then descend and later holding or descending to goal pace by #10 and then descending.

DON'T BE CONCERNED ABOUT WHAT APPEARS TO BE A LARGE AMOUNT OF REST (18 SECONDS AND MORE WHEN DESCENDING). At this point I am building the strength and the rhythm of the 1:12 pace. At the high point of training the swimmer is worn down and needs the rest. This gets more important as the swimmer gets older (over 35). An additional benefit of this is that it simulates the feeling of a good distance swim; it feels easy in the beginning and like hell at the end, even when you even pace. I would do a lot of work around that pace, I will send you some examples.

As the competition season approaches, I start to do swims with intervals on 1:25 and holding just under 1:13 for most of it but always trying to descend at the end. Now if I was 20 years old I would do sets on 1:20 giving me just over 5 seconds rest, but at 53 I can't recover well enough from swims this hard. I will also mix easy swims with goal pace swims, example: 5 x 400 on 1 min rest — #1at OD pace, #2 at 1 mile goal pace, #3 at OD pace, #4 at 1 mile goal pace #5 descend each 100 from OD past 1 mile pace.

As I taper, I will do some pace work with generous rest just to keep the rhythm. 100's on 1:40 hold 1:12. At the end of every practice and warm up before the race, I do some of these, but I always end it with a swim that is right on the nose. If I am doing 8 x 100 on 1:40 at goal pace and hit #'s 5 & 6 exactly, I am done.

As explained above, the swimmer needs to find the groove of the pace, but has to get the full feeling of what that pace feels like at different points. Always train for a negative split, even an even split feels like a negative split.

Lastly, on using this for open water triathlon swims.

I suggest that swimmers find a pace that they would use for something a bit longer than the actual swim. For an Olympic distance swim, something closer to OD than a one-mile pace. I would have them train descending the last half for a negative split. In the actual tri-swim, don't descend it. You'll come out of the water with a great time and feeling good. No need to overcook your swim as you still have the bike and run to look forward to.