Usually I'll wear the heels out when I know I'll be getting in and out of cars, walking no more than a few blocks, and not standing for more than maybe 2 hours at a stretch. When I shop at the mall, I typically wear flats, unless it's a really quick trip (which it never is!). When I'm going to be walking a lot, I wear flats.
I actually have this deal with myself, I get up in the morning and dress in exercise clothes. Then I am not allowed to take a shower and dress in "real" clothes until I've exercised. So (because I tend to procrastinate about exercising) I do spend a fair amount of my day in workout gear, not all gussied up.
In my 20s I wore heels CONSTANTLY and NON-STOP. This led to a lot of pain, and the bunion, and back problems, so I'm right there with you on this one, Maya!
My last bout with back problems was a few years ago - and during that time, I couldn't wear anything but orthopedic-quality shoes. No flats - not enough support. Since switching to telecommuting, and learning to baby my back, taking up yoga, etc, the problems got less and less. I have been able to re-introduce heels, but with judicious use, and I'm VERY picky about fit and comfort!!
If I do wear them all day in the office, on the rare occasion I have to go in, or for a night of dancing, I will then wear flats the next day. But no, I would not commute on foot in them and I would not wear them to run through an airport or to attend a trade show or to walk all over a cobblestone city sight-seeing or for a marathon day at the mall! I don't even wear them to run to the grocery store, because I think carrying heavy grocery bags in heels is a sure-fire prescription for back trouble.
Basically I've just learned to listen to my feet. If they get grumpy, it's time to give them a break and go for the flats - or better - fashionable sneaks. But I'm choosy about my flats too! I like flats that have arch support and some cushioning.
Oh, you'll love this one: when I met my DH, and was having all these back problems, he made a deal with me - every time I buy a pair of heels, he challenged me to ALSO buy a pair of flat shoes!! I loved that challenge, and of course, I rose to it. So I'd say my shoe wardrobe is about half and half.