I agree with Joshua. I've developed imports for a client with nearly a million inventory items that processes over 1.5 million SOP invoices a year, 500,000 AP transactions a year, and 500,000 PO Receipts a year.
The problems isn't so much your monthly import volume (except for the time it takes to post)--it's the amount of historical data that can build up after several years of high volume imports. If your database grows to several hundred gigabytes, things like Inquiries or historical reports will definitely start to take longer. Clients with really large databases sometimes implement a data archiving solution to pull that old historical data out of their main company DB to improve performance.