Ill give it atleast 10 days. Atleast in my experience, beans the germinate a little later than the others usually turns out to be a more indica dominant phenotype (if theyre are different phenotypes in the strain your growing). So im usually rutting for the runts lol.
I used to wait and wait and wait at times if something was slow or just never opened but over time I pretty much came to believe what you said, if it hasn't opened in 4 to 5 days it is highly unlikely they will after that. Seldom do I have a slow poke but if I do I will not give one more than 4 or 5 days now, likely only 4.