There is a way in Windows to know how long it has been since we turned on the computer for the last time, but it is a bit messy and convoluted; Therefore, it is much better to have this Windows Uptime.
Windows Uptime is a small tool, totally free and does not even require installation, which, when executed, will open a window where it informs you of the hours, minutes and seconds that makes the computer was last turned on; As simple and fast.
Of course, Windows Uptime is such a lightweight and basic utility that it hardly consumes system resources.
You can free download Windows Uptime and safe install the latest trial or new full version for Windows 10 (x32, 64 bit, 86) from the official site.
Devices: Desktop PC, Laptop (ASUS, HP, DELL, Acer, Lenovo, MSI), Ultrabook
OS: Professional, Enterprise, Education, Home Edition, versions: 1507, 1511, 1607, 1703, 1709, 1803, 1809