What are the highest practical rates at which men can increase muscle
mass, for example how long to add every 5 pounds of muscle mass,
through exercise? (No steroids or other funky supplements)

What are more reasonable rates that an "average guy" could increase
muscle mass? As just an example, one might assume a 45-year-old man,
currently slightly below average fitness, willing to dedicate
reasonable time at the gym with appropriate muscle-building exercises,
but has a full-time sedentary job and can't devote insane hours to
body-building. (Who me?)

Can an average guy with decent workouts add 5 pounds of muscle mass
every month? Every two weeks? Every three months? I suspect at some
point there would be diminishing additional returns, requiring ever
greater devotion to body-building.

adding muscle mass is relatively easy.  not adding fat mass is the hard part.

The Average amount of pure muscle a natural bodybuilder could achieve
in a year would be around 12 lbs, that does not include bodyfat which
is added in the bulk up process which is needed to gain body mass.
You should aim to gain 1-2 lbs in weight each week.
If you can devote 3 hours a week to the Gym, Monday Wednesday and
Friday.  Keep sessions heavy and under an hour but the most important
thing is to be consistent and eat every 2-3 hours consuming 5-6 meals
a day (more if necessary).
Eating at work may be a problem.  Meal replacement drinks will help
when your on the move and don't have time to eat.
Smaller more frequent meals are better than larger less frequent meals
as this will keep your body in a constant anabolic state and slow down
the chances of the body storing fat.

*People new to training will get more explosive gains in their first
six months so take advantage of it.

*Remember The diet is just as important as your training.

I have to disagree somewhat with the answers here. While it is true
that someone who has ALREADY gained a large amount of muscle mass
(such as a professional bodybuilder) will have slow, minimal gains in
lean muscle tissue, a beginner or someone with relatively low muscle
mass levels can experience gains in muscle mass that far surpass the
12 lb estimate per year. I have seen individuals gain 60, 70 even 100
lbs of muscle in a year. All naturally, without steroids or drugs.

What's the secret? Well first they were relatively new to training and
so they had a great potential for growth. Second, they took their diet
and training seriously. Anyone can gain a few pounds of muscle a year,
buy just training regularly and eating fairly well. Nothing
special...BUT if you want to make incredible gains in muscle mass then
you need to approach this seriously, sort of like you are in school
studying for a degree.

It's all about sacrifice. Everyone wants to build muscle, lose fat,
have a great body and a nice six-pack and bigger biceps, but they
aren't willing to put in the time and work required to actually do it!
 You can't expect to make incredible progress without some level of
incredible dedication and a incredible inconvenience.

Good luck!

Anthony Ellis
http://www.fastmusclegain.com
http://www.fatlosstips.com

I appreciate all the helpful comments. FWIW, for me personally I just
want modest muscle growth to facilitate modest (about 20-25 lb) fat
loss. A 60 lb gain in muscle is not in the cards for me!

My understanding is that building and maintaining each additional 5-10
lbs of muscle increases energy needs and can burn an extra pound of
fat per week. But it's an interesting curve, requiring initial weight
gain (in muscle growth) to assist longer-term weight loss (from
increased energy needs).

So the comments here give me a good guess of what the first part of
that weight curve can look like, if I persevere in my
decent-but-not-incredibly-dedicated-exercise ways. (And of course more
healthy eating is important too.)

Thanks.