In my diocese, it's a minimum of six months; some parishes make it nine. And you're expected to go to Pre-Cana (a workshop with compatibility profile) and Engaged Encounter (done as a weekend retreat).
Pastors have to insist on this; I'd imagine some parishes take it more seriously than others.
You're right.
My wife and I work with about five engaged couples a year, and, over the last five years, we've had serious reservations with three of them.
On two occasions, when I heard that "A just asked B to marry him," I actually choked on my beverage, because A and B were a recipe for divorce. (in one of those cases, she actually stormed out of Engaged Encounter after throwing the engagement ring at the fiance in front of the group.)
Better to throw an engagement ring before the marriage takes place than a legal brief afterwards!
We attended Engaged Encounter. It was considerably less than orthodox, which seemed cool at the time. We heard a few things we wanted to hear, and didn't hear a few things we needed to hear. In hindsight, I name at least 3 clearly erroneous messages that came out of it. I suspect it has changed for the better, but don't know for sure. We really saw it as a way to get of the more "burdensome" pre-Cana program. We also saw a couple split during EE, but we didn't have the ring-throwing.
BTW, to all those in the Engaged/Marriage Encounter/Retrouvaille movement...I salute you.