It can be instantly or in some cases a week or more.
Any offers that take a week or longer to credit have it
noted in the offer description.
Crediting time varies depending on the advertiser.
Helpful Tip: When signing up for surveys from the same company, make sure you clear your cookies in between each offer and use another e-mail address. Using an email address more than once for surveys from the same company will result in no credit being issued as they will see it as a duplicate signup. Make sure that the e-mail address belongs to you though and is legit. Some good email programs to use with offers is gmail.com or gmx.com