What Alveus said.
At my parish, there is an official service for the enrollment of the catechumen done before the Liturgy on Sunday. Generally, the catechumenate is a year.
I had a very long and involved inquiry (I was trying to convince my girlfriend at the time to convert with me, and didn't want to take any official steps for fear of further alienation). We broke up, though, so when that happened, I moved back into the area of my parish and requested official enrollment. I was enrolled the next Sunday, this last November (Sunday before St. Philip's Fast) and will have a shorter-than-usual catechumenate. I will be received into the Church by baptism on Palm Sunday. I was an inquirer for a little over a year prior to my enrollment into the catechumenate.
It's a shame that some churches do not have an official ritual for the enrollment of the catechumen. As baptism is union with the Church, like marriage (and, for those who do not know, there are many, MANY similarities between the baptism service and the marriage service) enrollment into the catechumenate is the betrothal. Traditionally, a catechumen who leaves the Church is considered apostate. If a catechumen dies, he or she receives an Orthodox funeral. The catechumenate is not an informal thing.