This is the type of question I get asked daily. There is no easy response. A response will require more information regarding the pair of shoes:
How many miles are on them?
Do you wear them daily?
When did you purchase them?
What do they feel like?
These are the type of questions I ask as I look at the shoes and place my hand inside to feel the cushioning near the forefoot. I look at wear patterns on the out sole. I look at the alignment component to see if it has broken down.
Here are a few ways to make your shoes last longer:
1. Track mileage, keep a log and record mileage, if you walk around in the shoes that equates to 4 miles / hour.
2. Have quiet feet while running. If you hear your shoes sliding in on the heel, or pushing off with your toes this puts substantial pressure on the foam and prematurely wears down the mid sole.
3. Run more efficiently. By working on your cadence,180 steps / min, you will require less out of your shoes. The longer your foot is in contact with the ground the more the mid sole is being compressed. This is where you can prematurely wear out mid soles.
4. Rotate shoes, if you are running 25 - 30+ miles a week get a second pair. Alternate your shoes. They will feel better and last slightly longer than if you wear one pair.
Some rules to follow for shoes in the $90-$140 range, lower priced shoes 1/2 the time
20 miles a week 4months
30 miles a week 3 months
40 miles a week 2 months
By following this simple rule you can run longer and feel better.
Of course it would be easier to just tell everyone that they need a new pair, since I own a running shoe store, but really it depends.