I don't think being strong and fit means you have to spend more time exercising, it just means you have to make sure the intensity of the exercise you do is high enough to challenge your body.
I'm going to give a fairly facile answer to your question: the right amount of exercise is the amount you will do regularly. I would love to do a one-hour workout 6 days a week, but given various constraints and all the other stuff I want to get done in a day, I never get there. I have a baseline that is a little lower, that alternates between short workout days (half an hour of cardio, no lifting) and long workout days (20 minutes of cardio, 40 minutes of lifting). As my level of fitness has improved, I've pushed myself harder during those workouts, not made the workouts longer.
I am sure that there are people who would be of the opinion that my routine is not "enough" but it's better than setting a routine that I am not prepared to accommodate and then skipping it half the time.
High weight: 275 (August 2009) *** Low weight: 155 (October 2012)
Today, working off a partial regain. Current weight: 175.
* Make the best choice I can make, with every choice.
* Remember that the temptation in front of me is not the last of its kind that I will ever see; say "I'll pass today."
* Say "no!" to my whiny inner five-year-old.