[nradov]

There's no way to know without testing. The optimal supplement amount for you could be zero, or it could be even more than 4000 IU per day. This depends on genetics, diet, ultraviolet light exposure, body composition, and a variety of other factors. So, the only way to be sure is to get periodic blood tests and titrate the supplements up or down to hit the target level.

If you don't want to hassle with testing then something like 600 IU will be adequate (although not necessarily optimal) for most adults. YMMV.

https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessiona...