I'll take a shot at this one but I'm way out of my depth here. Either way I will end up learning something new though so it's a win for me.
I couldn't really find out how much IU of insulin a normal healthy adult pancreas produces in a day because it seems to depend on a lot of factors, food intake seems to be one that would make a huge difference and since BB'ers are eating more calories / day than the average guy walking around, the amount of insulin produced I suspect is higher.
http://www.ncbi.nlm.nih.gov/pubmed/6993139
In the present study, we have determined prehepatic insulin production in six normal men throughout a day that included three typical 750-cal meals. Total insulin secretion for the 24 h was 45.4 U, secreted as 10.6 U with breakfast, 13.4 U with lunch, and 13.8 U with dinner. The remaining 7.6 U was secreted during the 9 h night at a rate of 0.85 U/h. At least 50% of the newly secreted insulin is known to be extracted by the liver during the initial transhepatic passage, so that total peripheral delivery can be estimated as approximately 22 U/day.
3x 750 calorie meals a day in healthy adult men produced 45 IU of insulin in a day. That's only 2250 calories. I would suspect that a BB'er eating 4000-4500 would at least produce double that?
I'm going to guess around 90IU daily would be endogenous production for someone eating that much food and to see a true anabolic effect from insulin alone the athlete would need to inject well over 100IU a day, probably closer to 200IU for a definite anabolic effect.
Am I way off here Jim? I want to know the answer but I figured you wanted someone to take a crack at guessing first, so as the least qualified guy here I felt the need to give my take.