I can't quantify how addicted I have been to this game, but for those who would be interested in reading a personal story in this subject I left a wall of text below so that you could have an idea about the extent of my addiction.
*Wall of text alert*
I started in 2004, but didn't play much until Ixion/Sandworm days. Until then, I was only using a mule character created under my brother's account who was a hardcore player. I was in university and my classes used to start at 5 pm. I used to come home late and wake up at noon, so I had time to play while he was already out for his own classes until around 2 am which was the time I had to leave home.
Despite all that, I didn't feel too bad that I didn't have an account myself. I took several long breaks as well, one of which took a year. The game wasn't that much attractive to me and we didn't have another PC anyway (playing on PS2 wasn't an option for us). I only managed to take whm to 75 and bst to 50 during that time. I chose those jobs for ease of getting invites and that I could do it solo in my very tight and random gaming time, respectively.
I didn't regret the time I spent to level up a mule character though. Meeting up with people from all around the world and sharing my time with them doing what we enjoy was so fun. It also helped me improve my English. This might sound strange, but it was a tool of socializing for me as well. This game provided me a place where I could talk to people I would never meet in real life while doing something I enjoy. I didn't need to worry about first impressions, appearence, lack of common interests to talk about etc. I have never been a social person, but I believe that FFXI didn't let me become even worse in that aspect.
In 2007 or 2008, I had my own laptop and that is when I started my own account and I have been playing almost non-stop since then. Before the release of Abyssea, our small group of fellow players quit, including my brother, but still, I never thought about leaving with them. I am pretty sure I am the only person from my country who still plays FFXI (please, prove me wrong). From 2008 (the year I graduated from university) to the late Abyssea era (when we had nothing better to do until delve), I was very active in end game scene from HNM camping to empyrean farming, staying up until up to 2 am (the pain of being an EU player). I knew it wasn't very healthy to stay up that late all the time, but accepted that as a humble sacrifice for a hobby.
Once I found a job, I finally stopped eventing in a NA-heavy LS due to time constraints and had to move to another server hoping to find an EU one where I was heavily disappointed in the end. About 2 months ago, I had to move again and since then, I am having a much better time.
tl,dr: I can't say I am proud of the fact that I have been playing this game for 11 years, but I can say that I don't regret it at all.