org-extra.el

(length
 (org-ql-query
   :select '(point)
   :from (org-agenda-files)
   :where
   '(and (todo)
         (not (or (ancestors (todo))
                  (scheduled)
                  (deadline)
                  (ts-active)))
         (let ((next-review (property "NEXT_REVIEW")))
           (and next-review
                (time-less-p
                 (org-encode-time (org-parse-time-string next-review))
                 (current-time)))))))