It can be any length, but it should not be done at all. It rarely works. The material, if it is necessary, should be integrated into the story which should start with chapter 1. Using a prologue is a cop out by a lazy writer. And many people don't read the prologue at all.
I know the question is months old but you probably didn't yet finish writing the book and you certainly haven't selected an answer, so here's mine.
Prologues are stories, and stories aren't bad. What matters is execution.
Let's see what makes prologues bad so we can think about overcoming those aspects.
prologues are boring if/when, instead of immersing ...