Yes I have had that happen.  I warm up on mats at our course on Mondays through Thursdays and on grass on Fridays thru Sundays.  The mats have a tendency to do a number on the paint and you can tell which clubs I warm up with.  I play about four days a week and hit my higher number clubs a lot so the paint is wearing off on those.  That has happened on every set I have had, regardless of the brand.  Of course, I am 100% Titleist now.  I now own a Sharpie paint stick and fill in the black when needed.  I do not think it is a matter of time as it is to how much you play with them and the conditions they are played on.     

Same with Chuck.  I have the Sharpie paint stick as well, and re-fill them as needed.  

The best way that I have found if you are paint filling and you want it to last put a layer of clear nail polish over the number and it will keep for longer 

Thank you for the replies and Sharpie recommendation everybody. 

On a side note, can't wait for the 716 AP2's to become available. Already checking to see how much I will get when I trade-in my current clubs to make room in the bag for them..

it's wear and tear

I actually took the paint fill out. Better to have it all gone than some with and some without.  I might repaint with a color for something different but now I'm gaming them paint-less.

I painted filled my scotty cameron neport 2 and I saved a lot of money and time. It only took me one day. I imagine irons are not as hard but might take longer. You need high quality acetone and paint remover, and some acryilic paint in the colors that you want. You might have to re-paintfill your irons if the paint comes off. Good luck and have fun with it!!