The old rule says restring as many times a year as you play per week: twice a week, twice a year. It is a reasonable floor and nothing more, because it was written when everybody played with natural gut or synthetic gut, which stay playable until they break.
Polyester does not work that way. It loses tension from the first hour and keeps going, and it goes dead long before it snaps — the bed stiffens, the ball starts landing short, and the player compensates by swinging harder, which is how arms get hurt. Fifteen to twenty hours of play is a realistic life for a polyester bed, whatever the string looks like.
Three signs it has gone: the ball lands shorter than you aimed with the same swing; the strings stop moving back into place after a topspin ball, so you find yourself straightening them between points; and the frame starts to feel harsh on contact.
If you play twice a week on polyester, that is a restring every couple of months. If that sounds expensive, it is the strongest argument there is for a multifilament: it holds tension far longer, and for most club players the difference in spin is smaller than the difference in cost.
One thing worth doing on the day it is strung: write the date and the tension on the butt cap in pencil. It costs nothing, and it turns "it feels a bit off" into a number you can act on.