Ideally, for RCIA one is supposed to spend a full liturgical year in the Catechumante period. That would be beginning of Lent - beginning of Lent, plus a bit before for inquiry and Lent - Easter as Elect. That would make it last for about 2 years total. The goal here is that you get to experience the whole liturgical cycle and that provides the back drop for all the basics that need to be covered, ie you learn about the Eucharist around Corpus Christi, the incarnation at Christmas, that Mary is without sin on the Immaculate Conception, that kind of thing.
That said, in MANY, if not most, parishes, the school year seems to be the norm, so September - May, so about 9 months.