How often to replace strings depends a lot on how much you shoot, what conditions you shoot in, and how well they are maintained (kept clean and waxed).
I know people that need to replace twice a year due to huge volume of shooting and have guys come in with bows 10 years old that look brand new. Unfortunately, you can't gauge the life of your strings only by looking at them. There can be breakdowns inside the servings, strands fraying, serving separations, or stretch and creep can become a problem resulting in incorrect draw weight or length or out of tune cams. The good news is that while expensive, technology in string components and string making processes make it likely that the new strings you buy today will be much higher quality than the ones that originally came on the bow. Peep rotation is a thing of the past with quality strings, no need for the old rubber tube on your peeps any more. I've seen many a bowstring replaced that looked fine but were definitely worn out, increased bow speed and better performance were the end result. My general rule of thumb to ask when people are looking at replacing a string that is a few years old is to ask how long they plan to keep the bow. If they say they will be shooting it for many years I would recommend a new set. If they plan to replace the bow fairly soon and they don't look bad, wax em up and keep shooting!
Also, please replace strings and cables as a set, while the string usually apppears to take the most wear, the cables are actually the working end of the equation and are typically under more stress than the string.