Currently I have a buffer of about 41.5 thousand words, which comes out to somewhere around 70 single-spaced pages. This means I'm 25 updates (a little over 12 weeks) ahead, and also have a good bit of bonus content already written. To be fair, I started posting with a buffer of about this size already built up, so I've just been putting in the work necessary to maintain it and edit existing writing.
I would actually like to be quite a bit further ahead. I'm pretty meticulous about foreshadowing (I often do sentence-level edits to set up for events several story arcs in advance) so having a long buffer is really important to me. Plus I'm petrified of letting the quality of my work significantly degrade, letting my schedule slip, or resorting to poor plotting in order to meet a deadline.
Honestly, I probably would be paranoid and feel behind even if the entire novel was already written, because that's just how I am.