Frequency

You should not lift with the same muscles more than every other day.  In other words, your muscles need at least a full day to recover fully, usually more.  You should also give your muscles a 2 day rest every week.  For example, a great routine would be to lift every Monday, Wednesday, and Friday.  You could work your upper body on Monday and Friday, and your lower body on Wednesday.  Preferably, you would be doing cardio on Tuesday, Thursday, and Saturday.  This would be an ideal situation if you want to be the healthiest possible in regard to exercise.

As important as rest is for muscles, if you don’t lift for more than about 7 days, you will begin to lose a little muscle mass.  I say begin, but really even after a few weeks you probably won’t lose much muscle, but you will lose a little.  Your body is sometimes too efficient, so it will eliminate the “extra” muscle if you don’t have a “need” for it.  When you lift frequently, your body sees the extra muscle as a necessity, but after a few days, it is treated as unnecessary.

Duration

As mentioned in the last section, strength training should last no more than 45 minutes (assuming your are doing it as described with intensity, rest periods, etc.).  Longer could counterproductive, and less than 30 minutes may not be enough to really achieve much benefit.  Typically when I lift, I do 5 exercises for 8 minutes each.  So I may start out with curls and do sets as outlined already for about 8 minutes.  Then I do the same thing for bench press, tricep extensions, etc.  When I’m done, I have been working out for about 45 minutes followed immediately by a whey protein shake (more on this in the next section).

Changing it up

The human body is amazingly adaptable, so it will adjust to your regimen after a while.  By changing your routine (not necessarily the muscles you work) you can get a more effective workout.  For example, sometimes you should use heavy weight (and consequently fewer reps) and sometimes lighter weight (and more reps).

You should also change the actual lifts you do about every 6 weeks.  For instance, when you work your shoulders you could lift a single bar with both hands, use barbells, or do handstand push-ups.  All of these exercises work your shoulder, but in slightly different ways.

You can vary the type of lift you do for any particular muscle as often as between workouts, but you should do it at least every 6 weeks.  If not, your muscles will adjust to the particular exercise so it will be difficult to get any stronger unless you force change by altering your routine.