She uses a Volkl Tour 9 V engine. 18x20
I restring it approx. every 6 weeks
she's a junior playing 14 and 16's and hits fairly hard
Thanks
OK, firstly totally no offence meant, if she is getting 60 hours out of a stringjob then she isn't really hitting hard enough to put much of a hurt on the frame.
Secondly and I am just making an assumption here (and we all know what happens when you assume) that she only has one frame. Which for any sort of tournament player is absolutely absurd.
Please peruse my handy guide to finding your ideal number of racquets. Located here.
http://tt.tennis-warehouse.com/showthread.php?t=127992
To answer your question, I would retire the frame after two years or so. You can only assume that as she gets bigger and stronger and better and starts hitting harder and breaking more string the racquet will take more of a beating. But who knows, maybe by then she will want to switch to something different.
The thing with the racquets getting soft is you don't notice it because you play it every day, and it happens really slowly. Like if you bought 2 brand new navy blue t-shirts. And you put one in your dresser drawer, and the other you wore and washed every day for a month. During the month you wouldn't notice that the shirt was fading, but if at the end of the month you went and held it up next to the brand new one you could see that the one that was washed 30 times was faded compared to the new one.
It is a good idea if you don't already do it, to keep track of the date of purchace of the racquet, and how many times it has been strung. Especially when you are dealing with a bunch of racquets. You don't want more stringjobs on one racquet than another, you want them all about the same so they wear evenly.
Here is how I keep track, but keep in mind that I am way more nuts about this stuff than anyone needs to be.
http://tt.tennis-warehouse.com/showthread.php?t=123530
J