I just went through changeing mine - twice so I can offer a few sugestions.
First if you do change it use GORE GFO - nothing else. I changed mine out last month while the boat was on the hard with the PTFE Flax packing from West Marine. Once the boat was in the water I could not after 2 and a half weeks get it to drip the proper amount and the stuffing box ran warm. I also read all of Mainsails website as well as other posts so I know I was doing the right thing but could not get it to function properly.
Finally I got so sick of it that I ordered the GORE GFO stuff and just changed it while the boat was in the water.Once the packing nut was off, sea water did rush in but no where near out of control enough where the bilge and pump couldn't handle it. Everyboat is different so whos to say yours won't come in quicker but for mine it was easily manageable.
With the GORE GFO packing I was able to tighten the packing nut 3 times as much than I was able to with the PTFE Flax, and get it to drip properly and run cooly right off the bat.
So...I can't really answer your question about knowing when to replace it on your boat but if you do, go with the GFO and attempt it on land but it should also be manageable while in the water as well but maybe not best for your first time changing it out.