The frequency of the surveys is highly individual. Moreover, it's better to check your e-mail too, as sometimes I've received surveys in the mail and they could not be seen in e-rewards site at that time. Updating your profile may help.
If nothing helps, your account may have been flagged as unhelpful/ giving random answers/ trying to guess the right answers, in which case you'll never get more surveys. Customer service will tell you if this is the case.