1.Make or reuse some script for monitoring disk usage, you can easily write based on df command : df -h <disk> grep -v Filesystem |awk '{print $5}'
this is just starting point.
2.Make some tresholds in script e.g if used is 80 % make output WARNING, if used is 90 % make output CRITICAL, so as you can see this is perfect place to use Nagios or some other monitoring tool with graphing usage so you can easily monitor per hour day, month, year. Laziest and best solution from my point of view
3.If you want custom solution, i will use cron for this, so i will make script with my tresholds and put it in cron to be exectued every hour,
crontab -e
* 0 * * * /path/to/script >> /some/file
This can be sent via email (if you have working MTA agent on machine) or script can generate some CSV report or simply make output of the script to be apended to some file with timestamps.