ST works through creating microscopic tears in the muscle fibers, which then grow back stronger. But it takes time for this to happen, so most experts recommend resting 48 hours between ST sessions (it is fine to do cardio in the meantime). Realistically, this implies 2-3 times per week.
Key concepts in ST are reps, and sets. A rep (repetition) is an individual exercise, incorporating the 'up' and 'down' movement. A set is a continual sequence of sets.
Intensity matters for ST, and you should be aiming to genuine challenge your muscles, and fatigue them with 8-12 reps with every set. Fatigue means you feel you cannot do another rep with the correct form. If you start having to jerk, lean, recruit other muscles to assist, etc, then you have reached the point of fatigue. And once you can do 12+ reps of an exercise, it is time to move up to a heavier weight/more challenging exercise. Although when starting out, you may want to go slightly lighter at 12-15 reps to learn the correct form.
You should be aiming at 2-3 sets of 8-12 reps, 2-3 times per week. You should also be aiming to work most of the major muscles in your body with each workout. Choosing compound exercises that work several different muscles simultaneously should enable you to get an all-body workout in just a few moves. An example of a compound based routine would be:
* pushups (modified, wall or incline pushups if necessary)
* pull-ups/lat pull downs/bent over dumbbell rows
Demos of these exercises (and many others) can be found at www.sparkpeople.com/resource/exercise_demo
'How long' isn't really a major factor in strength training - what matters is 'how challenging'. But realistically speaking, an all-body routine is going to take 20-40 minutes.