We always wanted to know how full memcached is, and therefore know at which age an element is dropped. This hacky script will find it out, by inserting 30 values each day and taking out 30 untouched values from 30 previous days <-> if one is missing, thats how old your oldest element is.
(if you know a better way, let me know 😉 )

Usage
Run one time each day (via cron) and store output into a logfile after 26 days:

rake check_memcached_age
Stats for 2009-12-01:
0: still there...
1: still there...
2: still there...
3: still there...
...
23: still there...
24: still there...
25: 
26: 
...

Your cache is 24 days old!

Script

task :check_memcached_age => :environment do
  cache = ActionController::Base.cache_store
  # insert probes for today
  (0..30).each do |i|
    cache.write "memcached-probe-#{Date.today.to_s(:db)}--#{i}", 'still there...'
  end

  # extract old probes
  results = (0..30).to_a.map do |i|
    [i, cache.read("memcached-probe-#{(Date.today-i.days).to_s(:db)}--#{i}")]
  end

  puts "stats for #{Date.today.to_s(:db)}"
  puts results.map{|day, present| "#{day}: #{present}"} * "\n"  
end