There are several stages of membership in the Lay Fraternities of Saint Dominic. The terms for the stages, and the duration of each stage, may vary somewhat from province to province; here I'll just use the terms and durations of my own province.
The first, informal stage is one of inquiry or postulancy, during which the inquirer or postulant participates in the life of a chapter for six to twelve months, long enough so that both the inquirer and the chapter are ready and willing to move on to the novitiate stage.
Becoming a novice involves being formally received into the Order of Preachers, and on the part of the novice a commitment to try living as a Lay Dominican for one year. It's a significant step -- once you're received, you're as much a Dominican as the Master of the Order. The novitiate is a time when you should be hoovering up knowledge of Dominican history, purpose, and way of life.*
If you're still on track (and both you and your chapter have to agree on that), then the next step is to make a temporary promise to "live according to the Rule of the Laity of Saint Dominic for three years."
Then, if three years later you and your chapter can still stand each other (I put this informally), you make your final or perpetual promise to "live according to the Rule of the Laity of Saint Dominic for my entire life." Once you've done that, watch out, because if you're not careful they'll elect you to the chapter council, or worse.
Postulancy or inquiry: 1 year (give or take)
Novitiate: 1 year
Temporary promise: 3 years
Perpetual promise: your entire [remaining] life
This doesn't mean, though, that after five years you're a fully formed Lay Dominican. No one is ever a fully formed Dominican.
For the friars, each province has a "promoter of permanent formation" and each priory has a "conventual lector" to make sure they're all always learning.**
Similarly, each chapter of Lay Dominicans is to offer ongoing formation to all of its members. This post was supposed to be about the nature of that formation, but I think it's gone on long enough already.
* Note how I switched from "the postulant" to "you," turning this didactic post into a subliminal recruiting tool.
** Or something like that. The friars have convents and priories and houses and who knows what all else, based on how many friars are there and such like, and there are different rules for each kind of domicile, and it seems like every time I try to make some categorical statement about what the friars do, one of them says, "Uh, yeah, we don't do that [here/any more]." If you want to take a stab at interpreting their Book of Constitutions and Ordinations, knock yourself out.