I got home the other evening from a five-day trip to San Francisco, where I had the pleasure of accompanying my wife on one of her telecommunications conference/business jaunts. I don't enjoy business travel, but I can tolerate what can be best described as "pleasure" travel; trips where I don't have to do too much save for drink beer, drink coffee, eat, exercise, read, and sleep. Those trips are usually "earned" by participating in a small amount of "sherpa" activity, carrying back extra or excess "stuff" in my luggage which probably shouldn't have been brought (or bought!) by my loving wife in the first place.
So, I often bring clothing items I can readily jettison on the last morning of the trip, which usually includes a single pair of running shoes at the end of their effective life span. I decided to sacrifice my oldest pair of shoes on travel after seeing one too many shoeless indigents walking down Caroline Street in Key West, FL. To a homeless person, a beat-up pair of running shoes is better than no shoes at all. I try to clean the shoes a day or two before the end of trip and leave a note for the housekeeping staff. The act of perhaps helping another person in a world of hurt by giving them something as simple as a pair of old shoes also helps me try to not hurt myself by running in a pair of running shoes which is too old.
Every so often I get an athlete who complains of sore feet, ankles, shins, knees...sometimes even sore hips and low back. I immediately look to their shoes to see their condition, then ask the question most runners cannot answer: 'how long - in miles & months - have you had those shoes?" If a runner has pain & their shoes have less than 300 miles/six months then the root cause is either something biomechanical, something overuse, or they're in a pair of shoes which isn't right for their foot type. But it's more likely that the runner cannot tell me how many miles or how many months they have had that particular shoe...which usually means they've had the shoe too long.
Most shoes will last an average of 400 miles or eight months, whichever comes first; a ballpark figure which has wiggle room on either side based on the runner's weight, weather conditions, sweat rate & terrain choice. Runners (like me) who sweat a great deal & run in humid climates go through shoes at varying rates throughout the year; shoes purchased in October or November last a few miles/weeks longer than shoes purchased in March. Lighter, less-sweaty runners can almost figure out down to the week when they'll need a new pair. I keep track of the cumulative mileage for each pair of running shoes I have - I rotate between three pairs because I do sweat - on my training log spreadsheet. If I start feeling an ache in the foot or ankle I can look at the mileage & determine whether the cause is training volume/intensity or if it's time to look for a new pair of shoes.
There are several ways to tell whether a pair of shoes is near the end of their lifespan. The most humorous, & probably the most noxious, sign your shoes are dead is when they literally smell dead. I've taken a "truly dead" pair into a local running shoe store to get a replacement, & they begged me to leave the shoes outside at the curb in a biohazard baggie. But that's really the most extreme case of zombie shoe you should ever see. Personally, I hope you never go that long between shoe purchases.
The biomechanically-challenged runner will wear the outer sole rubber off at the places we used to wear out our old Converse Chuck Taylors, at the heels or the balls of their feet. I've seen some shoes with the outer sole under the big toe worn smooth. Typical runners, however, will have crushed mid-sole, where all that nice, white foamy EVA has serious crinkling along the sides of the shoe. You can also tell if the midsole is dying or dead by taking the sockliner (another term for the shoe's insoles) out & giving the EVA a push with your fingertip. If there's no give when you push or rebound when you release the midsole is probably dead.
Lastly, when the EVA looks & feels good and the outer sole looks like a champ, take a close look at the places along the shoe where the upper portion of the shoe joins with the sole. If there is a break between the shoe upper & the sole, the chances are high it's time to replace the shoe. Those are usually the most heart-rending replacement times for a guy like me, because I've just become happiest with the fit & feel of the shoe. But, regardless of whatever emotional attachment you might have with that old shoe, when it says "it's not you but them", it's time for them to leave. New sockliners (insoles) should only be placed into those old running shoes if you plan to use them for kick-arounds, gardening, or lawn-mowing. Or if you're one of those folks who likes your workplace footwear to say, "yes, I am a runner," or, "yes, I like to get paid to do things while wearing ugly running shoes."
But a job where you can get away with being paid while wearing ugly running shoes is a beautiful thing. Ask any coach.