I do not agree with the 10%
I do not agree with the 10% rule; and I have 2 stress fractures to prove it.
Generally, people mean 10% increase in mileage per week. The time period is important because you get very different results if you say 10% per month, 10% per week or 10% per run. 10% per week. If you increase, 10% per week, you end up increasing almost 50% per month. So, if you are running 15 miles a week, you'll be running 32 miles a week after just two months. That is definitely too much too soon.
In 2007 I was increasing less than 10% per week and I still cracked my fibual (I was running about 50 miles a week). In November I was increasing by about 10% per week, and I got a stress fracture in a metatarsal (this was caused, however, in part by some crazy downhill speed running on one evening).
If you are recovering from a stress fracture, I would increase by max 5% per week. I would actually recommend keep the mileage low and constant, and increase first the number of times per week you run. You need to test first whether the stress fracture has really healed.
I am just getting back to running myself after the above-mentioned injury. My plan is to run 3 mi. 2 times per week. If that works out, I will add in week 4 a 3rd run of 2 miles. If that works out, I will then increase that distance in week 6 to 3 miles. After that I will go to 4 times a week, but keep the mileage low. If you calculate it out, each of these increases is more than 10% individually, but they are much less per week, since I only increase every 2nd or 3rd week.
Cheers
Paleo