Dragondoor article says 60s.
Book says 15s.
Basically, it doesn't really matter.... as long as you can proceed to the next progression WITH GOOD FORM and hold it for 3-5s for at least 3 sets I would move up. The one caveat is that the next progression is not taxing your joints or tendons too much causing some form of overuse injury.
That's my take on statics at least... seems to agree with the book.