Good morning and welcome to Android Central. I have an S6 Edge+ and this function is not available on my device. I do believe there was a wifi timer setting on older versions of Android. I am currently on 6.0.1. There is a setting to switch wifi on or off when the device is in sleep mode. To access this setting, do the following.
1. Pull down notification shade.
2. Tap the settings icon.
3. Tap the wifi setting.
4. Tap the "More" setting in the top right corner of the screen.
5. Go in to the "Advanced" settings.
6. About mid way down you will see a setting labeled "Keep wifi on during sleep".
This is not exactly what you asked, but it's as close as stock Android 6+ offers.
The good news is there are 3rd party apps available in the Google Play Store to help you with your question. Please let us know if we were able to help you.
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