This completely depends upon the user - how often they use the tablet - how hard they press with the nib - the age of the tablet surface.
The new(er) tablet surfaces have a bit of a "tooth" to them. This does tend to chew through nibs much faster than the surfaces used to.
When I used an Intuos 2-4 the nib would last months, if not years. With the new "textured" surface of the Intuos 5 I found I was replacing nibs every month or so - but I use my tablet daily 100% of the time. I've never used an Intuos Pro. I have an Intuos 5 touch (which is basically the same as a "pro".) But I imagine the pro has the same textured surface as the 5 does.
Eventually the texture surface will get worn down to a smooth surface (similar to old Wacom surfaces) simply due to use. This tends to let the nib last much, much, much longer.
Wacom apparently changed the surfaces because users were commenting on not liking the glass-like smooth surface. I, personally, prefer the smooth, glass-like surface. With the Intuos 5, if you like the textured, nib-eating, surface it takes a warranty repair to replace the surface. This was a very non-consumer friendly decision by Wacom. It would probably be better to get used to the warn, smooth surface, than to send your tablet in for 10-20 days to get the surface replaced, and that's if it is still under warranty.
Nibs are relatively cheap, if you feel like the nib is scratching the tablet, it's time to replace it. How often you need to replace the nib is entirely dependent upon your use.