Hi Lebensbild,
Welcome to Microsoft Community.
I'm Hahn and I'm here to help you with your concern.
You need to press Windows Key + R and type gpedit.msc to open the Group Policy Editor if you are using a Windows 10 Professional system, then follow the steps provided by JananiDhana, an independent advisor.
You can modify the registry as follows if you are using Windows 10 Home Edition:
Open the Registry Editor and visit: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System, create a new 32-bit DWORD Value named InactivityTimeoutSecs, and set the value to 0.
Disclaimer: Generally, modifying registry subkeys or work group is intended for advanced users, administrators, and IT Professionals. It can help fix some problems, however, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For further protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click here to view the article.
If there is anything not clear, please do not hesitate to let me know.
Your Sincerely
Hahn - MSFT | Microsoft Community Support Specialist