I actually registered an account soon after discovering the Farms because I wanted to be able to rate posts. I was only planning on lurking but then @Dynastia noticed me spam-rating his posts (as I was going through the infamous Golden Knight thread) and struck up a conversation; he ended up convincing me to start posting, which I did about a week after I registered. I've never looked back
It's tough to put a number on the length of time between starting to lurk and forming an account. I've known about the CWCki Forums for years but just posted on /cwc/ from about late 2009 until 789chan closed. I lurked the forums for a while from 2012-ish on but fell for the idea that CF couldn't begin to touch /cwc/'s deep discussion of lolcows. That community started going to shit around the time of the transition from /cwc/ to /cow/ and I started reading the CWCki Forums exclusively when /cow/ moved to 8ch.net. I finally broke down and made an account after the GameStop incident.