Performance Monitoring - frequency?

This is a sheer preference issue. You first need to determine how critical that monitoring is for each server and then adjust your polling frequency to match. How soon after something happens do you need to know about it? Within 15 seconds or 15 minutes?

The easiest way to do this is create three or more separate monitoring instances (three consoles) called, for example, High, Medium, and Low Priority. Set the polling rate for each of these groups accordingly.

You can also break apart the services and what you are monitoring in a similar way. If you are tracking number of connected users, is that a stat you want hourly? Are you checking available disk space daily? Again, these can be separate monitors such as Hourly, Daily and Weekly.

The more you dump into that database the more you can track, however it is also more that you have to sift through to find what you are looking for.