find out outlook was open for how much time using .net application.

In addition to what LNogueira already said, you can use the WaitForSingleObject Function (Windows)[^] from the Win32 API (you can use it directly from C++ or by marshalling from C# and VB).

If you pass the process handle of outlook.exe to this function, with the second parameter equal to INFINITE (INFINITE is defined in WinBase.h as 0xFFFFFFFF), the function does not return until the specified process terminates (it doesn't matter how it terminates).

Then you can work this way:

1. Test if the process outlook.exe is running
2. If outlook.exe is not running wait for a while then return to the step (1); otherwise go on with the step (3)
3. Save the current time (this is the time when outlook is started)
4. Get the handle of the outlook.exe process
5. Call WaitForSingleObject to wait until the outlook.exe process terminates
6. Get the current time and update your statistics
7. Return to the step (1)