Part of the issue with vitamin supplements is that the bioavailability can be unpredictable. The actual amount absorbed can vary between 10% and 100% depending on the time of day, supplement formulation, what foods if any are taken together, as well as the particular characteristics of the individual's intestines, which are difficult to assess. Because supplements are not regulated as pharmaceuticals in the United States, this variability can be severe; in the worst cases, supplements do not even contain the active principle.

So, I am not surprised that someone needs to take 5000 IU to get 600 IU worth of effect. Institutional medical authorities are (rationally) quite defensive when cautioning readers about supplement consumption; they must consider the worst case (100% bioavailability) when assessing the risk of overdose.

As an alternative to vitamin supplements, exposing common dietary mushrooms to ultraviolet light converts (by an uncatalysed photochemical reaction) the ergosterol therein to calciferol. How best to achieve this in a home setting is unclear.

One number is not going to work for everyone. The only way to be sure is to get a blood test for Vitamin D levels. I get tested with my yearly physical, but if someone really cares they can get more frequent blood tests.

As has been commented elsewhere, everyone absorbs vitamin D differently, this really is a matter where someone should just get tested, if they (and their doctor) decide supplementation is needed, do so, test again, and adjust dosage accordingly until desired levels are attained.

Not medical advice here, but harmful effects from vitamin D exposure/toxicity generally only happen at very high levels, or if high doses are taken over long periods of time (as excess can be stored in fatty tissue/liver). Doctors often prescribe a very high dose (like 50,000 IUs) for individuals who are very deficient (often taken once a week, not daily) for a short period before going on a more standard (400-2,000, maybe 5,000) IU dose for maintenance.

The advice on this is all over the map and that's a big problem in the space. Reputable medical sources have recommendations almost two orders of magnitude off from each other at times.

This article, for example:

https://www.ccjm.org/content/89/3/154

...cites several cases where daily supplementation of 50K IU was required to restore normal D levels, although also a case where that same dose caused toxicity. As one of the other commenters in the thread noted, working with your doctor to establish the right level is probably the right move. If nothing else, they have the capability to test your serum levels to see where you're at.

I experimented with taking 10,000 IU a day for about a year. I had my D levels checked with my normal yearly blood test (lipids, etc) and it put me into the high normal range. I still take 5000 IU daily and have for years with no ill effects.

I should note that I live in a place that sees little sun for five or so months a year.

You can't possibly have the same recommendation for all geographies. Florida and Scotland have somewhat different level of UVB especially throughout the winter, come on.