By default it takes real 96 minutes for 24 in-game hours to pass in-game hours in Witcher 3. For me it's way to fast. Fortunately, it is really easy to fix, it's literally changing one value in r4Game.ws

The line we need to change is:
SetHoursPerMinute(0.25);
Here are some values you can be interested in:

SetHoursPerMinute(0.133333);    // 1 hour = 7,5 min,25 hours = 180 min = 3 hours SetHoursPerMinute(0.1);         // 1 hour = 10 min, 24 hours = 240 min = 4 hours SetHoursPerMinute(0.066666);    // 1 hour = 15 min, 24 hours = 360 min = 6 hours SetHoursPerMinute(0.05);        // 1 hour = 20 min, 24 hours = 480 min = 8 hours SetHoursPerMinute(0.033333);    // 1 hour = 30 min, 24 hours = 720 min = 12 hours SetHoursPerMinute(0.016666);    // 1 hour = 60 min, 24 hours = 1440 min = 24 hours (duh)

But if you're too lazy to do it yourself or want more "modular" approach to modding your game, just pick one of options from the download section and place it in your mods folder, or use your mod manager of choice to do it for you. The default version is 4 hours.

This mod was inspired by Slower Gametime 1.31 GoG GotY mod that is not compatible with Witcher 3 next-gen update.
I didn't want to wait until it's fixed, so I made my own version. Enjoy.