I usually have 8-900 km in pair of shoes...

After 100 km I measure the sole depth and when it is 10-20 % less - usually 3-4 mm after 8-900 km - I start looking for new shoes...

Very unscientific... but it works for me :-)

*EDIT*: 
By the way: 
Basically I use a normal ruler. I put in inside the shoe and measure the hight to the next shelf in my bookcase... Easy :-) I do this every 100 km or so.

![My shoe in the bookcase][1]

Note that I usually apply a little bit of pressure on the ruler to make sure my custom made insole in properly positioned.

Also I don't measure anything the first 100 km to allow the shoe to "settle in" first..

  [1]: https://i.sstatic.net/fF8MW.jpg