The rails file cache is something that keeps growing.
Here are some 'handy' snippets.

Delete all files older then 7 days. (replace -delete with -print to test it)

find ./tmp/cache/ -type f -mtime +7 -delete

Delete all empty folders. (mindepth prevents the deletion of the root folder)

find ./tmp/cache/ -type d -empty -delete -mindepth 1

The whenever.rb I use

job_type :command_in_path, "cd :path && :task :output"

every :day, at: "0:40" do
  command_in_path "find ./tmp/cache/ -type f -mtime +7 -delete"
end

every :day, at: "1:10" do
  command_in_path "find ./tmp/cache/ -type d -empty -delete -mindepth 1"
end