Someone asked this question on the Steam thread a few days ago, so I'll c&p the reply they were given here:
The best way to answer this is by checking out the last jam: https://itch.io/jam/tyranojam15 and the winners: http://tyranobuilder.com/summer-jam-15-winners/ we held a much stricter 30min rule so all the games you can play there can give you an idea about how much context can be fit into 30mins.
Generally speaking you want a given ending to be 30mins but the reason for the time limit is set is so our guest judges can easiely get to all games and can make a verdict, so if you are making multiple routes I highly suggest you put just as much effort into one as you do the other as they may not be able to cover every route and will likely base their impression on their first playthrough.