This is going to vary from 14 year old to 14 year old. AND, MOST IMPORTANTLY, any increases in mileage should be done gradually. That said, I really think more in terms of how long your "basic" typical run should be. For my recommendation, I have simply worked backwards from the load I think a serious senior distance runner in high school should run. I think a typical day for a serious senior is 8-10 Miles a day. There are kids that will run more, and obviously a lot that run less. From that, I simply work backwards and try to adjust the mileage gradually so there are no big jumps. So, working backwards, I'd go 7-9 for a Jr. 6-8 for a Sophomore, 5-7 as a Freshman, and 4-6 as an 8th grader. Again, these numbers are what I would recommend for your basic bread and butter staple run-- which does not mean you multiply this number time 7 automatically to get weekly mileage. There will be days that exceed the mileage, days off, and eventually maybe 2-a-days. This is the format I have used to train my 14-yr-old son, and he has been very healthy and remained enthusiastic. I also advocate at least a few weeks completely off after seasons at your age.