It all depends where you applied. Less competitive places generally send out more invites and earlier, some people have invites within a week of submitting the application.
Places that know they only need to interview 50-100 applicants bide their time, wait to see what their potential pool looks like before sending out most of their invites.
Like everyone else who as been through the process, you initially want any interview you can get, then get too many invites and cancel everything in the end as you grow tired of interviewing.