IMHO it's not the amount of @replys... It's the total amount of spam reports you get. Spacing them out doesn't matter if you still get the same amount of spam reports - the accounts may last longer, but you're still only getting out the same amount of @replys out, and thus the same amount of traffic.
However tons of spam reports in a short period of time definitely seems to bump you up in the queue to get reviewed and banned. It does appear to be a manual review - you will notice that when you are spamming outside office hours in the continental US, your accounts will stay up for significantly longer (but there are also less people online to see your tweets).
You're also going to get more complaints if the list of people you're @reply-ing aren't targeted somewhat to the offer you're promoting.
Dupe urls and exact dupe tweets are also a definite no-no. Unique-ify your urls, even if it's just adding a random query string to the end. You can see a few sites out there that have the most-tweeted urls for the day - you definitely
don't want any of your urls to be showing up on those lists.
IMHO you have to choose between doing hardcore @reply spam or doing "legit" accounts with ad tweets (not @replys) mixed in with lots of real tweets.
I personally get out 200-400 @replys before I get banned, so you guys might want to tweak your technique a bit. I see some people getting out even more.
