That sounds about right.  They will only sit on the nest for that long if they are broody.  Do you have a broody?  If you have any doubts about whether you have one or not, then you don't have one.  Broody hens are pretty distinctive in their behavior.  I had never seen a broody hen before last year.  My girls are not prone to going broody, but last year one went broody on me.  It took about 2 seconds of watching her behavior to say "Oh... so that's what everyone was talking about when they said 'broody'."