This is personal opinion
Common User: 8gb
Now pc consumes a lot of ram, the ram is cheap but sometimes we cant put more for the motherboards.
RAM is not the only component that drives the speed of a desktop or laptop computer. RAM is only memory. I'd say your hard disk and how it has been set up would be a very important determinant of speed. Particularly if you got SSD. Then processor speed is for me the most important determinant of your computer speed. A computer's processor clock speed determines how quickly the central processing unit (CPU) can retrieve and interpret instructions. This helps your computer complete more tasks by getting them done faster. Clock speeds are measured in gigahertz (GHz), with a higher number equating to higher clock speed.
4 GB at least for the very basic usage.
8 GB for normal usage and normal games.
16 GB for some serious apps/ games.
Also, keep in mind that RAM is not the only that matters, processor and motherboard should be according to the RAM you are using.
For standard users
4 GB will be succifient on windows7
6-8 Gb is good for windows 8,8.1
8 Gb is good for windows 10
On unix/linux side
if it has GUI(Graphical User Interface-> Window systems) then it should start 3gb
if it has only terminal, it could be succifient 1gb but 2gb is good in my opinion