Direct mail response rates vs # times mailed

[Jeff Fairchild]

If a person sends out 1,000 mailers per month for 6 months and they are expecting a 3% average return, is that 3% each time they send out a mailer or 3% overall for the 6 months?

So is that 30 calls over 6 months or 30 calls per month?

I've heard that it is best to vary the types of pieces, ex. yellow letter then postcard then zip letter etc. If i do this should i be expecting more than 3% overall if I'm mailing 6 times?