On the main screen of the app, select the light, then tap on settings (the cog icon). This will open the settings of your light, where you can edit both “Fade-in”, which is how long it takes for the light to reach full brightness, and “Fade-out”, which is how long it takes for the light to turn off.

Note: Fade-in and Fade-out only apply when lights are used with the WiZ app and WiZmote.