there is no real set time for replacing the props ,on any drone ,its all down to where you fly, how you fly ,and whether or not, the props actually make contact with something, either during flight, or during landing on uneven ground ,as long as you check the condition of the props ,before every flight ,visually and by feel ,(dont forget the underside where the lugs lock it to the motor)
also check the moving pivot point between the prop blade and center hub for wear and tear
of course you are at liberty to replace them as often as you like ,if you so choose
beware of cheap knock offs on the web ,Master Air Screw ,generally tend to be well made ,and are favoured by many pilots ,as a replacement for OEM props