I get the following question a lot "when should I change exercises, in my workout routine?". I normally answer if the exercise you are doing is still producing gains in strength and muscle size you should keep doing it. And if the exercise has gone stale, meaning you are no longer getting gains from it, then it's time to change .
I also recently found this interesting study.
Research has shown that performing the same weight training exercises for an extended period of time may inhibit strength and muscle mass gains. Researchers tested individuals performing biceps curls for six weeks to determine whether or not the amounts of muscle involved when performing curls changes throughout an exercise cycle. By the end of the training cycle, the amount of muscles recruited during the bicep curl decreased by 20%, even though the amount of weight used increased. These results provide further evidence to show that in order to meet your fitness goals; the weight programs should be changed at least every six weeks. This is most easily performed using a periodized weight-training program, where the exercises performed, as well as frequency, volume, and intensity is routinely manipulated.
M. Bernardi, et. Al., "Motor Recruitment Strategy Changes With Skill Acquisition," European Journal of Applied Physiology 74(1996): 52-59.