How to set how much time the notification have to be shown?
Ubuntu uses Notify OSD, which does not allow you to control the timeout. The timeout duration depends on the length of the message.
Where is a complete pynotify documentation?
On Ubuntu, pynotify is provided by the python-notify package.
According to /usr/share/doc/python-notify/copyright, the package contents come from the galago-project.
The latest tarball was on 2006-10-08, and contains no documentation.
The closest thing I could find to documentation was the "Desktop Notification Specification".
Can the notification be clickable? (example: if I click on the notification, print "hello world" in the terminal).
Unfortunately, no. See "Avoiding actions".