There are all sort of options. Check the docs or code. Try this:
http://jsfiddle.net/aamir/cbB5P/1/
$('.flexslider').flexslider({
slideshow: true,
slideshowSpeed: 2000,
animationSpeed: 400
})
Check more options here: https://github.com/woothemes/FlexSlider/blob/master/jquery.flexslider.js#L1047