How can I make my risotto less firm?
Time has nothing to do with cooking risotto -- you cook it 'til it stops absorbing liquid.  I don't even cook by amount of liquid added, as it's more a cook-it-until-it's-done type recipe.   When the rice stops aborbing liquid well, it's generally done, but an occassional check for texture never hurts.

Type of rice will affect things, as will elevation from sea level, temperature of the cooking vessel, and surface area of the cooking vessel.

See : 


Why do I need more time and liquid than my risotto recipe calls for?
How should I prepare Risotto