I'm no expert on the matter but a couple thoughts for you. The benefit of IF is that the body goes into a kind of detox mode that allows for cellular repair and regeneration. This quote is from a study published by the American Journal of Clinical Nutrition on alternate day fasting (a technique where people ate 25% of their caloric intake on the fasting day, then normal amounts of food on the second day):
In terms of diabetes risk, animal studies of ADF find lower diabetes incidence and lower fasting glucose and insulin concentrations, effects that are comparable to those of CR. Human trials to date have reported greater insulin-mediated glucose uptake but no effect on fasting glucose or insulin concentrations. In terms of cardiovascular disease risk, animal ADF data show lower total cholesterol and triacylglycerol concentrations, a lower heart rate, improved cardiac response to myocardial infarction, and lower blood pressure. The limited human evidence suggests higher HDL-cholesterol concentrations and lower triacylglycerol concentrations but no effect on blood pressure. In terms of cancer risk, there is no human evidence to date, yet animal studies found decreases in lymphoma incidence, longer survival after tumor inoculation, and lower rates of proliferation of several cell types. The findings in animals suggest that ADF may effectively modulate several risk factors, thereby preventing chronic disease, and that ADF may modulate disease risk to an extent similar to that of CR. More research is required to establish definitively the consequences of ADF.
Mark posted on the article and sums it up here:
A recent article in the American Journal of Clinical Nutrition gives a great overview of these benefits which include decreases in blood pressure, reduction in oxidative damage to lipids, protein and DNA, improvement in insulin sensitivity and glucose uptake, as well as decreases in fat mass.
So a longer duration without food definitely has some benefits, but you are nursing so now maybe isn't the time to push it. I really don't know whether a shorter period of 'detox mode' would affect your nursing but a longer one might. It would seem logical that a longer period of 'detox' would mean the body might reduce milk production. Also, Martin of Lean Gains recommends slightly shorter fasting periods for women compared to men (14 hours vs 16 hours) but I don't have the quote; it is something to do with differences in glycogen depletion. If you want to try it, maybe just start with skipping breakfast, then eat as usual from noon to evening or whatever window fits best with your schedule. Also read up on the Lean Gains site; a few people on this board have also recommended this protocol. You can adjust the diet recommendations to more primal eating. He pretty much dispels the myth that skipping breakfast lowers metabolism; and a lot of people find that breakfast is the easiest meal to skip. You can also get by with some coffee/tea with a spoon of coconut cream or milk. Skipping breakfast doesn't slow metabolism in the least; also keep in mind that Martin (and others who IF) are getting a full supply of calories in the eating window. Skipping or even just delaying breakfast makes it easy to get a 14 hour fast (say last food 8 pm, then first food at 10 am; noon would be 16 hours) might be all that is appropriate right now. Also I have read that after about 17 hours most of the benefits are in. Definitely break the fast with a high protein and perhaps some carb meal. If you really feel good on that and it hasn't impacted your nursing you could extend to the 16+ hours one day a week (like a weekend) when you have more time to rest anyway. But start slower and see how it affects you. It takes a few days for the body to get used to a certain eating time and also get used to the idea of going for longer stretches of time without food. Bottom line just try a shorter fast for a few days or a week and see how it feels. Lastly, a note from personal experience, make sure to drink more water when you go without food. The body gets some of its water from food and when we skip a meal we still could use the water.