If the Antiochians in the US are instructed to baptise after 6 months from FIRST attendance then that is not a lengthy catechumenate.
That is the MINIMUM ("at least six months"). And there is a difference between the length of catechumenate and how long you're attending before being made a catechumen. I have friends at an Antiochian parish where the time as a catechumen is 12-18 *months* but the length of time a person attends before they're made a catechumen differs. The Antiochian parish where I was chrismated had a priest that required a minimum of six months' attendance before you became a catechumen. I was chrismated six months longer.
My current OCA parish makes folks catechumens a bit sooner, usually after several months of regular attendance and regular catechesis. The actual catechesis program is about nine months, allowing for gaps due to holidays, Holy Week/Pascha, etc.
And if you stop attending/classes due to being unsure, crisis of faith, etc., you might pick up where you left off with the classes, but you'll be a catechumen longer than you would have originally been, since obviously there are issues that need to be dealt with.
While these are general guidelines, in some cases people go longer before chrismation. Some just aren't sure once finishing their formal catechesis is done.