Hi Zdenek,
Raw data in ArcGIS Monitor is continually aggregated and automatically truncated when more than 14 days old. Only aggregate data (day, month, year) is stored after 14 days. A big portion of the storage space is due to the raw data, so the recommended way to limit the size and truncate old data is as follows:
- Adjust the sample intervals from 5 minutes to 15 minutes or 1 hour to reduce raw data input
- In Administrator, delete unnecessary counters, focus on Production environment
- After 14 days, the old raw data will be automatically truncated
Also, there's a FAQ about projected storage space growth, where it mentions "Once you have 14 days of data, a growth rate of 1 GB per month is typical." So, I suggest to plan for this. A couple users have requested a setting to control when data is automatically truncated and an enhancement request was already logged.