While creating vm using Nova boot I am supplying it a user-data script 
(cloud-init script). 

what I am wondering is, how can I specify in that cloud init script( or any other way to do it) :- to schedule a cron job to run every 2 hours 

lets say I want to run "du -s njain/ " 
to find the size of my directory every 2 hours 

I know "runcmd" can be used to do it once..but how do i make it run periodically ?
 du -s ../njain/