The rest of the crew may chime in and offer tips too, but there are a variety of reasons, or indicators, of when it's time to change your string.
Many players will describe their poly strings as feeling "dead" (when initially they felt very "lively") after a few weeks of using it. This is a big indicator that your strings aren't holding their tension and are causing you to do more work to achieve the same results. Even though the string is still useable, this extra work can cause arm aggravation.
There are also tools that, when used properly (extra emphasis here), can measure the total stringbed tension over the life of the string. http://www.tennis-warehouse.com/desc...PCODE=STRMETER
The tension that is read will typically be lower than the tension at which it was originally strung, but it will provide information on how much tension your racquet is losing over time.
Again, these are just some suggestions but we hope they help you in the future.
TW Customer Service