Ask HN: What are your monitoring data retention policies, and why?

I'm in the process of putting in place a monitoring solution which, to begin with, will collection information on:

* CPU, RAM, disk, and networking; * The system's load (averages, # of procs, etc); * User information (logged in, and so); * HAProxy, MariaDB, MongoDB, ElasticSearch;

I'm wondering what a good retention policy would be for a generic monitoring solution like this? And Why?

What policies does your business mandate and what is the reasoning behind them?