As with all else concerning SQL Server, "It Depends".  Some subjects can be adequately covered in a lightning round in only 10 or 15 minutes.  Sometimes, even shorter.  Other's aren't adequately covered even as a precon, never mind a 60 minute or even 90 minute session.

Shifting gears to the subject at hand, I think a little research by the people setting up the conference would go a long way, especially in drawing people to present.  To me, it seems that 1 hour sessions with a 15 minute break between sessions seems to be the norm in the world of SQL Server thanks to things like SQL Saturdays and the PASS Summit.  Deviating from that "norm" is going to cause presenters that frequent the conference circuit to scramble to reduce the usually carefully planned timing of their presentations, remove important material from their presentations, or do that all on the fly when a conference deviates from the implied norm of 60 minutes to a 45 or 50 minute schedule.

I'll also say that having a homogeneous session length also seems to be the norm and certainly makes it easy on both the schedulers and the people attending.  While I'd love to have 90 minute sessions for what I teach, a mix of 90 minute and 60 minute sessions would be a problem for the attendees.  For example, if you attend a 90 minute session and then want to attend a 60 minute session, you're total wait time between sessions could be the half hour length offset plus the 15 minute dwell time between the hour long sessions for a total of 45 minutes.  That can be a total waste of time.

As to the question of "How long is too long?", I'd again say "It Depends".  If the presenter is an idiot or a bore, a 10 minute lightning round can be painfully long.