Writing.Com General Discussion

Depends entirely upon whom you are writing the premise for.
If it is for a pitch, the shorter the better. If it's for a query letter, 2/3 lines seems about right.
And if it's for a course of study, they expect a whole paragraph! (yeah... that's what I'm stuck with)
But if it is for you alone to give your book something upon which to hang your planning, then it's up to you.
Nothing hard and fast - know your audience.

To be honest, the only time I've ever written one is for my current Bachelor of Arts degree...

If it's a precis (as opposed to a premise), no more than a paragraph. In general.