I would see it as a statement of commitment - and intention to marry at some future date, which will vary from couple to couple depending on circumstances.
Engagemenets are like pieces of string - there is no set length.
I've been with my missus nearly 22 years - can't remember how long we've been engaged, but it's well over 10 years.
My dad was with my stepmum nearly 26 years before they got married, so who knows - maybe it's hereditary ???
I've known two couples who were engaged for years but were divorced within twelve months of the wedding. I could never understand why they went through with it, suppose the relationship must have been failing so finally getting wed was a attempt to patch things up.
What's an engagement really? Just an agreement to get married. There's no time limit on it. I knew someone who'd been engaged for 18 years. Haven't seen her for ages. I often wonder if they're married yet. Doubt it.