How often should one change oil in a a car that is all highway driving?

Make, model, year, monthly mileage and where you live all can change the answer.

All highway driving should follow the recommended NORMAL oil change
schedule as recommended based on make model etc.  City driving should
follow the SEVERE service change recommendations.

The answer is not as simple as it used to be! Many oil manufacturers
(mostly trhe synthetic guys) are touting obscenely high oil change
inervals.  Of course the old "pat answer" used to be every 3000 miles.
I suspect in your case that would have you changing oil every time you
turned around!

A few basics:
Oil should be changed for several reasons: first, oil will break down
chemically over time and lose its lubrication properties. 
Multiviscosity oils can lose their ability to flow more freely (than
single viscosity oils) in cold weather. All oils (single and multi
viscosity) can "thin out" with time and lessen their ability to
lubricate.  Heat accelerates these changes, therefore oil would last
longer in a moderate climate than in a hot one.

The reason synthetic oils last longer is they have a "more pure and
stable" molecular structure that is more resistive to breakdown than
traditionaly refined oils.

As an oil breaks down it combines with combustion chamber residue from
blowby gasses (for definition of blowby see my article
http://econofix.com/pcv.html) to form sludge.  High detergent oils
dissolve this sludge and hold it in suspension.  In addition, as an
engine wears small pieces of metal, rubber, and plastic end up in the
bottom of the oil pan.  Believe it or not, the "granny car" which is
"short tripped" will have a lot more acids (condensed from blowby
gasses) mixed with the oil than a car which is driven every day!  I
rebuilt one engine with a thrown rod: (old lady, once a week to the
store 3 miles away) The rod bearing had been eaten away by acid
deposits!!!  Ony 40,000 miles on that motor! Thus on low mileage cars
I recommend changing the oil every 6 months, no matter how many miles
on them!

So anyway, the oil should be changed to get rid of all the "crap" that
gets dissolved in it, even if the oil itself is still chemically OK.

MY ANSWER TO YOUR QUESTION:  (if you're not totally bored yet:)

I don't go by either miles or time on my oil in my cars! I go by
"feel" and "appearance"!

When I check my oil (you should do this regularly: how regularly
depends on how much oil your car uses: You'll "get a feel" for its
"oil habit!")  I wipe the dipstick off with my fingers.  I then rub my
fingers together and "feel" the oil's viscosity.  Good oil feels
slick: you can't feel the ridges in your fingerprint.  Really bad oil
feels almost like water: your fingerprint ridges are really obvious. 
"Feel" the oil right after you change it: if it feels thinner than
that, change it!

Step 2: reinsert the dipstick, pull it out to check the level.  You
should be able to read the markings on the dipstick through the oil. 
The oil should be a light caramel color: about like pancake syrup at
the very darkest. If it is darker than that, it's time for a change.

OK, let's say you're a fanatic with a lot of money and want to change
your oil every 3000 miles whether you need it or not: is there any
reason not to do this?

Yes there is! Each time you change the oil the oil pump "loses its
prime" ad the new oil filter is empty.  Even if you pour some oil in
the new filter you'll leave an air bubble in there.  When you start
the engine, it will have to run for a short time without oil pressure:
that's an automotive heart attack! (see my article:
http://econofix.com/oilite.html) Before the oil can reach the
bearings, the air must be forced out through the oil passages, and air
doesn't lubricate motors very well!

This is really not a huge big deal: the film on the bearings works for
a surprising length of time even with low oil pressure, but why do it
any more than you have to?

Hope this has helped!
George

Oh, one more thing: if there is a warranty on your car you should
follow the recommended oil change interval in your owner's manual, or
it may void the warranty!

The vehicle manufacturer specifies which grade of oil should be used
for the vehicles it produces. The manufacturer also specifies how
often the oil changes should be made. For example, most people believe
that a common oil change frequency should be every 3000 miles or every
3 months, whichever comes sooner. This 3000 mile oil change interval
has been relentlessly promoted by oil changing companies for decades.
It had a scientific basis when engines used non-multi-weight,
non-detergent oil. It no longer has any scientific basis, but it is
still being promoted by certain entities, most notably the oil change
industry in the United States (including car dealerships). Indeed,
studies have shown more wear occurs with fresher (1000-2000 mile) oil.
This is attributed to additives re-establishing themselves, TBN
converging, and filters becoming more efficient. Most manufacturers
recommend oil change intervals of 6,000 miles or more for modern cars.
For convenience, the oil filter is usually also replaced at the time
the oil is changed. There are many types (or sizes) of oil filters for
vehicle engines. Vendors of oil filters have information on which type
of oil filter is compatible with a given vehicle.
http://en.wikipedia.org/wiki/Motor_oil

Highway driven or not, consult your car manufacturer. Depending on the
make and model, each car uses specific oil and needs to be changed at
specific intervals regardless of miles and operating temperature.