Supposedly toxicity symptoms start around a blood level of 200 (I forget the units right now).
I've seen one patient taking 4000 units a day get her level to 106, she's been the outlier. Most people taking 5k/day get between 50-80 (which is where I want them), some need up to 10k to get there. This is why testing and re-testing is important.
I'm not so sure about people mega-dosing D3 every time they get a cold (flu, yes). With some people who get colds often, massive D3 dosing anytime they feel something could possibly get to toxicity over time. Seen this pattern before.