How can I get total credit note amount for a specific period?
The easiest way is to go to the Credit Notes tab and export. When exporting, copy only those entries in the date range you want. Paste into a spreadsheet and add a SUM.
The more complex method is to construct a custom report, querying the general ledger transactions table. But you will then have to add filters for transaction type and date range. Then you will probably want to group by reference number to get totals for each credit note. You will have to go through this for every different time period. But you will have a report that can be called up, cloned, copied, etc.