I assume you're concerned about falling asleep while it is streaming. If that's not your concern, then just get in the habit of returning to the Roku home screen when done watching -- the Home screen uses very nearly zero data. If you're really tight on data usage, don't use a Roku screen saver that needs to download currrent info (weather, sports scores, news headlings, etc.) although the data for this is minimal.
I know of no way to shut down the wifi other than killing the power to the Roku. To do this on a fixed schedule, you could power it via an inexpensive timer.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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