How long should a synopsis be?

How long should a synopsis be?

Derek Johns (Managing Director - AP Watt Literary Agency) gives expert video advice on: How can i make sure an agent has really read my work?; Who will read my work?; How can I sell an unusual or complicated story to an agent in only three chapters? and more...

I don't think a synopsis should be more than three pages long. I think it should be brief and concise, and it should operate with the early chapters of the book. An agent is looking to see where a book is going, but also to see what happens on the page in the first few chapters.