The vitamin D council recommends for cold and flu, 1,000iu D3 per pound of body weight for 3-5 days. I have safely taken as much as 300,000iu daily for 5 days without any adverse affects. When my family gets sick, I dole out 100,000-300,000iu D3 depending on who is sick -- my child or husband. I know he's deficient. I had him on 50,000iu daily (which he took sporadically) and when he had his levels tested a few months later, it was 32ng/ml. Imagine how low it was before he started taking 50,000iu. It was probably really, really low.
My brother, who scoffed at me, had his levels tested a couple of years ago and his level was so low it was undetectable. He regularly gets outside and thought his levels were just fine. We live in Virginia.
Over on the D3/sunlight forum, Chrisb has posted a study on diabetes type 2 patients taking 50,000iu daily and the finding is that the more overweight you are the higher dose you need, especially if you have problems like diabetes.