Well, 500 pages is about tops for a young adult book but don't go below 175 pages. Also, be sure to know what kind of audience your book will interest. Because that may play a part in the length.
I think it should be more than 250 pages (give or take a few) as long as the book will be interesting throughout all of the pages. For people like me, there is no such thing as too many pages, so if you can stretch the story and the plot- go for it!
