I just replace mine every 2 years along with my irons. I like all  my stuff matching, i'm weird like that.

If you're concerned about spin and stopping the ball up close, try switching to the ProV1 if you're playing surlyn cover balls. It was like I had new super wedges when i made the switch.

I replace my wedges once I notice the grooves becoming slightly worn.  Everyone hits different amounts of balls on ranges etc... so it will be a difficult challenge to make a general rule...Some tour players I have spoken with replace wedges every 60-90 days in some cases especially before major tournaments.  I personally replace wedges every year however I hit a lot of balls during the season.  

Great discussion, there was an in-depth analysis on the loss of spin by worn grooves on wedges recently on a golf website and the loss of spin was significant over time.  

Your Vokey Wedges should be replaced when the wear on the face begins to impact the ball spin and therefore your play.  

For me it varies by wedge.  I replace my PW every 3-4 years, my SW every 1-1.5 years, and my LW every 2 years.  The SW gets replaced most often since it takes the biggest beating from traps.

I hope this helps!

I agree with the two year rule. Really all depends on the amount you play. Keep your old wedges and use those at the range and save the good grooves for the course