Direct from Allakhazam, Warchief Tyrandor has put together an incredible guide on Scenarios .  So far he has written information about scenarios from Tier 1 and 2. 

All scenario share 2 common rules.

A race to 500 points: The first team to reach 500 points win.

A 15 minute time limit: If neither team has reached 500 points before 15 minutes, then the team with the highest score wins.

We've also posted this guide in our wiki .  Feel free to contribute by updating the guide with your own facts and additions.