This is a question that we get a lot over here at thecrossbowstore.com. The answer is that it depends. If you wax your crossbow string and and lube your rail then there is no reason that you should not get several years of use from the original string. Many shooters get at least 100 shots under their belt.
Putting the crossbow lube and wax
on will lower the friction that the crossbows tend to produce. It does depend however how often you shoot your bow during the year. You should always check for fraying of the string every time before and after you shoot.