Change waiting time for new user 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.
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.