i would say consult a doctor. if they think it is musclar they would send you to a physio to help you out .
I did buy new trainers about a year ago though.
while there's not enough research on how long you can keep wearing your trainers before they wear out , i am sure my physio said between 200-300 miles.
how much you weigh has alot of bearing on it too. the more you weigh the faster the cushioning will wear out obviously.
a rule of thumb is how you feel. if u feel that the bounce is gone and the cushing is not that amazing , get rid of the shoes. if you exercise , then roughly every 3 months.
it can be costy .... however, you can buy the shoes , but you cant buy healthy legs...
what i do is go to shops like nike , or adidas outlets and get them for a fraction of what they cost. 30-50£ ... which is pretty amazing considering buying the same trainers for 130£ at the high street shop!!!