Importantly , is there any statistics on typical %age of storage space saved when we archive a standard SAP FICO system, like 20 % , 30 % ?
--> There is nothing like statistics for % of space gained from Archiving. Lots of factors affects this statistics %, like total DB size, Number of open documents, period for Archiving , archiving objects you choose, proportion of FICO data in total DB size. So in generic i can say it would be 20% max gain.
Assuming the current growth of an FICO system is 50 GB per month , if archive is switched on , what should be the growth rate ?
--> Again it depends on how many years of data you archive and how much data you regain in DB, Your DB growth may increase in due course of time and the data which you were archiving for the earlier years data may didnt have much db growth. So assume you may reduce 10% in the total db growth.