"How many classes should it take for me to
get promoted to each belt?  I have a friend whose instructor told
him exactly when he should expect to be promoted to each rank.  I
think that's a great idea for an instructor to let his student know
when they'll be getting promoted.  What's your opinion on it?"

If that approach works for the instructor and he's able to produce
quality students that meet his standards, that's fine.  But I don't
think that should be used as a rule or expected from all
instructors.  In case you didn't know, it took me 9.5 years to get
my black belt (2 yrs white, 2 yrs blue, 4 years purple - my
hardheaded years, and 13 mos brown).  Doing that entire time, I
never asked Lloyd Irvin when I was getting promoted and I never saw
or heard Lloyd ask his instructor (Leo Dalla) when he was getting
promoted.  Dalla passed on to Lloyd (and he passed it on to us),
the concept of improving our jiu-jitsu and spend less time worrying
about when the belt promotions would be given to us.  So, that's
what I did and I think that approach is definitely considered "old
school" these day. 

Nowadays, with the "Americanization" of grappling, students don't
think twice about asking when they're going to get their belt
stripes or promotions (even when their grappling technique sucks)
as if it's their right to know the exact moment it's supposed to
happen.  They want to be assured of the payoff before the work is
put in on the mat.  And, unfortunately, some instructors have
catered to their students for business sake.  At the same time, I
don't think it's right for instructors to hold students back at
certain ranks for non-performance reasons (like to have a top
competitor be held at a certain weight division to help teams win
tournaments), but it does happen and probably always will happen
because some think that's the way it's supposed to be.  That's the
other end of the "When to Promote" Debate that leads to schools
having 4 year white belts that beat up visiting blue and purple
belts because their skill level doesn't match their belt level.

Belt ranking has become a motivational tool for people to train
harder and that can be a bad thing, especially when the ranks don't
come quickly and people feel like they're wasting their time
training since they're not being rewarded. People should train hard
because they want to perfect the art, not just to get a promotion
so they can get on the grappling forums and brag about how fast
they're moving up the grappling ladder.  Each grappler has to ask
themselves if the reason they started grappling was for the
knowledge or the belt.  And for the majority (in my opinion), they
grapple for the belt.  I really believe if ten people had the
chance to train exclusively with Rickson Gracie with only one belt
promotion (from white to black belt) and no timetable when they
would get promoted, at least 7 of them would decline the offer and
go somewhere where they could get stripes and belts to "let them
know where they're at" on the mat.