How long to lay an egg?

Hi all,
Just a quick question - how long does it take on average for a hen to lay an egg.  Rosie one of my chooks seems to be sitting for a long time again - about 2 hours so far.  Is this normal?  Should I be worried?  She seems perky enough, rushed out of the coop with the others for breakfast this morning - but was sitting in the coop last night for a couple of hours before bedtime.  Trying to make sure she got a comfortable nest box perhaps?