There will definitely be an effect, and within two weeks it'll be decently pronounced. A 'sider going on the Duskling diet takes a little longer to start feeling the difference than investigators eating 'sider food, since the investigators are a blank slate and 'siders have been eating this stuff all their lives, but going on the Duskling diet effectively makes them Dusklings, given enough time.

In the first week of his experiment, Almast won't notice major changes. By the second, though, his vision will be slowly starting to even out across light levels.