how seriously do you play? how many racquets do you rotate (if any)? what type of ball do you hit and how hard do you swing?
if you are a serious player then every little advantage helps, and i would restring no less than every 3-4 weeks. i play 6 times a week with two matches per week and restring about every 2 weeks due to tension loss. many of my teammates have to restring about every week due to breaking strings so quickly (even when using poly!).
if you are a recreational player and aren't playing any tournies or matches then i don't think it really matters. you will notice major tension loss and a lot of string movement. if that bothers you, then restring. if it doesn't bother you, then wait until they break or until they feel totally dead (2-3 months maybe?).
your setup sounds like a good one that should hold decent tension, and you probably don't need to be stringing more than once a month if you play 3 times a week and are a recreational player who doesn't rotate multiple racquets.