I need to consume 4000 IU/day just to keep my serum levels acceptable. I do agree though that blood tests are necessary. When I started getting into the vitamin D supplements, I had my blood levels tested every 3 months for a few years. I started out dangerously deficient and even 8000 IU/day was barely moving the needle in the beginning. However over a longer period 8000 IU/day ended up being too much, so I cut it down to 4000 IU/day where I've been for years now.
There are a lot of factors going into this and everyone's circumstances are different. Frequent testing to verify assumptions is a must.
I spend a good amount of time outside, going running regularly, and am in decent shape. I also eat 250g of salmon weekly, which contains about 1000-2500 IU. However at the end of the day I live in the north (59 deg lat) and the sun here is of limited impact.