May i offer a suggestion i heard from another game (i think Magic). have round time be 75min + 2 d 12 dice, only the Tournament organizer knows the d12 time.
once that time is up all mages get finite life instantly (imagine poision blood enchantment has been cast), and all damage done to mages form all sources from the next turn onwards is doubled (ie calculate damage normally with dice, pearcing etc, then just double that number)