How long is a pair of men's leather dress shoes supposed to last if
worn every day?  Also, how long before the rubber soles of the shoe
crack and why does this happen?  Thank you.

Hello, I don't think there could be any type of set time. just wear
them until they look worn, then buy new ones. Also you should consider
saving for two pairs, as no one should wear the same shoes seven days
a week. That will also add to the live of each pair. It's a good
fasion move to own a black, brown, and blue pair.

I agree with daizy1-ga. You can wear them until they look worn and you
will decide how worn your shoes are.
For example, I usually wear my leather shoes about 1-2 years. I have
only one pair (I disagree with daizy1-ga at this point) which usually
worn about 10-12 hours every day (on Friday and Saturday they are worn
about 14-16 hours). Yes! Seven days a week.
In general after 9-10 months they look a little worn, but it depends
how hours you wear them, how long you walk in them and which your
walking way is. Also, it is related to your feet sweat production. For
example, my feet usually sweat a lot, so the insides of my shoes show
(and is) wet. This has another problem, which is the foot odour. The
wet enviornment of the insides of the shoes usually products a strong
odour. Of course, you can reduce it with odour-eaters and other
similar products. However, I do not use any this product, so my shoes
usually stink or reek, but it is not a problem for me.
In conclusion, the leather shoes can be worn until their owner wants.
For example, a lawer should have unworn shoes, but a university
student (like) has any problem if his leather shoes show really worn.