It will most likely depend on what slider plugin, widget or theme, but you might try going to Appearance > Theme options > Slider Settings and see if there’s a place to change it there, that’s where mine is.
slide transition speed (ms) = 500
time between slide animation (ms) = 5000
Also found it in /wp-content/themes/YourTheme/js/jquery.nivo.slider.pack.js: animSpeed:500,pauseTime:5000
