The registration of my website works by user email verification.
How can I change a waiting time of new user activation? I have many spam users that register at my website; they register and never activate the account.
The project LoginToboggan has a feature to auto-cancel these after a preset time. From the LoginToboggan features list:
Optionally have unvalidated users purged from the system at a pre-defined interval (please read the CAVEATS section of INSTALL.txt for important information on configuring this feature!).
For more ideas about how to deal with spam registrations, see: How to deal with unverified users in Drupal?
In addition to what @Gisle Hannemyr said, I would recommend the spamicide module (sets hidden field that if it is filled by a bot on registration, it will not create the account) and the re-captcha module on your registration forms.