The length of the photography session really depends on who is being photographed.
As a general guide, we recommend you allow 45 minutes to an hour for your family's studio portrait session, which gives you the time to have a relaxed and fun session.
A photographer that promises a 2 hour sessions isn't going to capture that much more. It just means that you will be at the session for as longer time. If you have teenagers that aren't keen to be there this can make it seem like a very long session :) Little ones also have a very short attention span, so anymore than an hour is a long time for them as well.
We recommend not worrying too much about the length of session, but rather looking at the portfolio of the photographer and making sure you love the images they are creating.
It's also important that you arrive at the scheduled time for your session. Most busy photography studios will have multiple family sessions booked on that day. They will have an allowance for people running late, but if you are late it will impact on the time they have to spend with your family.