I purchased my 2003 C36 in the summer of 2003. The boat has always been in fresh water. I have never changed the pencil anode. The original anode was zinc. So, for some unknown reason, after all it's only been a little more than 16 years, I decided to change the pencil zinc anode on my M35B with the proper magnesium anode. I suspected that the zinc anode was probably asleep. The photos attached show the zinc oxide that forms when a zinc anode goes to sleep. Also think about this, after 16 years of use (350 hours on the engine) the zinc anode is pretty much intact and asleep. 
 
This leads me to the discussion of sacrificial anodes on boats. There is only one way to know if your underwater metals are being properly protected and that is the use of a silver/silver-chloride (Ag/AgCl) reference electrode for performing corrosion potential. ABYC recommends this; 
RECOMMENDED RANGE OF CATHODIC PROTECTION BASED ON AG/AGCL REFERENCE CELL
HULL MATERIAL MILLIVOLT RANGE
Fiberglass -550 to –1100
Wood -550 to –600
Aluminum -950 to –1100
Steel -850 to –1100
Non-metallic w/Aluminum Drives -950 to -1100
NOTE: Exceeding -600mV on a wooden boat may lead to significant damage to wetted wood surfaces in contact with the cathode.