Basically, it depends on the size of your environment. The bigger your environment, the more storage you will need for vcops.
That can be measured in either "VMs being monitored" (if we're talking about pure-virtual environment),
or in "metrics being collected".
The second is more-accurate from vcops point of view.
Another parameter is the data retention period, which is pre-configured differently on the vApp vs Standalone (as far as I remember it's 6months on the vApp)