It depends on what you're using it for. Unless you need the bleeding edge of new stories, 30-45 minutes between each scrape is probably fine. If you're looking to stay on top of 'new', your best bet is about every 10 minutes.
More frequent than that, and you're just causing trouble for the server with no real benefit for yourself.
Paul already provides data dumps. Searchyc for it.
I wonder if that is pretty standard? Do most services throttle at 3 seconds or is that being overly strict?
Most are only throttled by the bandwidth at either end.