If you use different vault stores for Exchange and any other archiving you are doing, you best bet is to run a 'count' against the 'archive' table in the directory database, at the beginning of each month, with a 'where' variable on the 'vaultstoreentryid' that relates to that vault store id that contains the exchange mailbox archives.
that way you get a count for all archives in the vault store, subtract form previous month and hey presto alakazam, hence why you need to have a different vault store for exchange and all the others.