Is there any method to know how much time a computer is used by someone?

According to me there is no way or method to know how much time a computer is used by someone but if you buying any second hand computer, then you can make a guess by the condition of the computer how old it is?

Sadly enough, I could not find any such way to find the time spent on your computer by someone else.
If you really wish to monitor someone using your computer, you may consider using a spy cam or such a device.

If you can keep a password on your PC, please do. This way no unauthorized access will be available to anyone else. If you can't keep a password on your PC, I suggest installing programs like TeamVNC, pair it up with a mobile app and monitor any usage that happens on your PC in your absence.

There are softwares that can count the time person has used the computer.

For example Convergys (tech support company) that keeps log of the person logged into the computer and logged out from it. It also keeps logs of the lunch breaks and the distraction time like meetings. Another such type of software is used in Netcafe where the login and the logout time is computed and the user is charged respectively. Upwork (formerly odesk and elance) has this software that you can download and use to keep track of time used on computer. This software logs the working hours of the employee.

There are some shell scripts that keep log of the keystrokes and the navigation. So if in your absence if the computer is used then all you have to do is check such logs.

Your problem is very specific and requires custom software development for you to keep track of time spent by employee. I suggest instead of using off the shelf software for this (which is going to cost you a lot by the way) use the custom made software.

I personally suggest running a shell script that keeps the log of keystrokes and the mouse navigation. So this way you can see what people are accessing in your absence.

If it is the case, you may then protect system by setting and securing through Password. If you think it is not advisable then you may try Rescue time software which needs to download and run securely in the background. You may refer https://www.rescuetime.com/

After a long time search on Web, I haven't found any method to find how much time a computer is used by someone! All you can do is just check all the parts of that computer, if they are in working condition then its all OK. Or you can also check the manufactures date on various components of your system. But sadly enough, I can't find a direct way to your question. As checking the used time period for electronics products are quite difficult. All you can check is its working condition to conclude how long it has been used by someone.