Maybe a stupid question, but what is the typical life of a set of brake poads? 80% city and 20% highway driving. Front would seem to wear out sooner. Is there an expected life/miles for the pads?

There are always wide variations in pad life based upon pad type, driver style, and percentage of highway driving vs. city driving. I've had pads last nearly 90,000 miles and other pads be done before they hit 30,000 miles. The condition of your rotors will also be a factor - if they're scarred up, they'll eat your pads quicker....

On our S-Type the factory front pads were used up at 32,000 miles while the factory rear pads were replaced a couple of weeks ago at 62,000 miles although they still had some life left in them. Our car is driven rather conservatively and does about 75% highway miles....

Jon covers it well. Driving style and terrain are big factors. Here where there are lots of hills, I expect to go through more brake material than you would if you lived in a flat area like Florida.

My front STR pads were swapped out at 27k, had a little life left. Rears are fine now, but being changed due to the dust factor.

Best way to figure it out for your personal variables, is to pull some wheels and take a look.