The general answer to this question is: it depends. Some of the factors which can impact how often you change the spark plug are how often you ride your bike, the distance you cover, the quality of the plug you have chosen, and the age of your motorcycle. As a general rule of thumb, new spark plugs should last around 6,000-7,000km. However, it is worth inspecting your spark plugs on a regular basis, as well as checking that all-important gap. You can also perform some basic maintenance including cleaning off any carbon deposits.
Another way that you can tell whether or not a spark plug is still in good working order is the color. If it is light tan or gray, this is a way of telling that everything is A-OK. If you notice that it is a heavy black color with deposits, it could be too cold for the engine. Alternatively, it could mean that there is too much of a spark gap. On the other hand, if you see a bright white color, it may be that the plug has overheated from improper tightening.