I'll change strings when I think I hear a sound quality change. That can be less than optimal; however, because sound change kinda creeps up on me. By the time I notice strings don't sound right it can be well past an appropriate change date. I wake up and, "Hey... these sound crappy!"

A better way for me is too change strings based on the time played. Not calendar time of weeks or months, but clock time of hours. Since my practice regimen includes rough lesson plans which in turn involves logging time played, I always have an accurate account of how many hours the stings have been played.

Like manXcat, I wipe down the strings after every session with an added extra cleaning at the end of the week. (The String Cleaner http://thestringcleaner.com/ ) Most of my time is spent on my acoustic and the practice and play certainly isn't thrash... 100 hours on a set of strings is more than enough if I don't hear the need before then.