I can sometimes feel a difference within hours of taking the methylselenocysteine form; however I mostly take se yeast. In the Winter it warms my typically cold hands.
I don't have a serious thyroid problem. My tests are still normal although not optimal.
I wonder if those who take more than a day to feel the difference have a problem with RT3.
Another possibility is that people who don't feel the effect quickly have a substantial amount of heavy metals in their blood. Se binds nicely with many HMs. While this helps to protect the body from the oxidative stress it would likely leave less Se available for T4-T3 conversion.
I've done years of heavy metal chelation--Cutler-after getting my silver-mercury fillings removed. I also don't consume much high Hg fish. So I should not have much HMs in the blood.