I wouldn't recommend editing in one timeline for a 90-minute project. Not because it isn't doable, but it is easier to manage and visualize the over arching story if you divide the project into segments/sequences. In my experience, the point at which you'd cut to commercial would mark the end of a segment/sequence. Make sense?
I wouldn't ever work in a timeline that is 12-hours long either, haha. The only time I have scene that is when I was an assistant and editors were pulling selects into a timeline.
Anyway, glad I could help you and your friend.
Rob Grauert, Jr.
http://www.robgrauert.com
command-r.tumblr.com