When does anode require replacement?

[boatdude100]

New to me boat last year 3L GL with SX-A drive with 2 anodes
Both are badly corroded which as far as I can tell means either...
1) They need to be replaced or...
2) they are working well...

Researched for an hour and still don't understand how I can tell when they need to be replaced?? They don't appear to be actually getting smaller. ( I think)
Note: Drive has some corrosion, and anodes seem to be magnesium boat run in fresh water
Thanks!!

 

[alldodge]

Both are badly corroded which as far as I can tell means either...
1) They need to be replaced or...
2) they are working well...

Both are correct
Anodes should be replaced when around 25 to 30 % are gone

 

[tpenfield]

Badly corroded/depleted means they are working and should be replaced.

 

[Scott Danforth]

If in doubt..... replace them

 

[Horigan]

Make sure the anodes are well bonded, electrically, to the drive. Them not getting smaller is a potential sign they aren't bonded well to the drive.