Google Hangout time out limit
For some reason a Google+ Hangout will time out after 90 minutes, unless you confirm to Google that you are still using it.
This is a shame as it stops ideas like having a support hangout open all day with moderators dipping in every so often to check for questions. Just be aware that Hangouts have limitations, the 90 minutes time out and a maximum of 10 participants. But then it is a free service!