The question of dose is a big one, but I personally don't go over 1500 IU of supplements a day (try the 1000 IU gummies IMO). I also only take it from November to February. The recommended daily dose is way less, and you can easily sit down and pop 20,000 IU of the stuff. Be careful! It is technically a hormone.
[1] https://www.sciencedaily.com/releases/2015/03/150317122458.h...
[2] https://pubmed.ncbi.nlm.nih.gov/28768407/ (two years later)
"... in a correct analysis of the data used by the Institute of Medicine, it was found that 8895 IU/d was needed for 97.5% of individuals to achieve values ≥50 nmol/L."
That just means there are some people who need a very high dose. For comparison, 4000IU/d gets me well over 50nmol/L, this is also the "generally considered safe" upper limit. There was a case reported on HN where 5000IU/daily caused issues with calcium levels. You gotta get your bloodwork done.
In my personal example, taking about 6000 UI (plus 300 mcg K2) for a year put me just under 200 nmol/l. I did an experiment and stopped taking it for 2 months (but kept taking K2). After testing again I had dropped to 70 nmol/l (top of the insufficient range) and had mild symptoms of muscle ache and fatigue. Needless to say, I've started taking it again (and will aim for 175 nmol/l).
There's just a whole lot of variables. If you're going really high-dose, check with your doctor. Even if you do need a high dose to catch up and level out, how would you know when to stop?
Search the string "devaboone" here on HN for more about the potential drawbacks of really high doses.