I've got original strings on my hunting bow since 1999. I don't shoot it a whole bunch and the strings are in great shape still. My tournament bow is another monster, I shoot it a bunch more that the other so I've all ready put two new systems on it. So, changing your string out will primarily dependon the wear and tear you do to it frmo lots of shooting. If you've got some broken strands, change it, excessive wear on any serving-check under the serving and reserve if you can-otherwise change the string. Also, like stated above, if you are twisting your string alot to keep the bow specs in, change it-you don't want to over twist your string this could result in the string snapping from to much stress on it.