This is a question that I get a lot. It's probably because I'm pretty adamant about saying that if you're only going to mail once don't mail at all.
I believe it takes time for people to notice you. That's why I believe in narrowing the list you mail to (there's an entire approach to this) and mailing several times to that list.
We're gearing up for a big direct mail campaign for our new Miramar campus and we'll hit the list we have 3 times within the 2 weeks before the first service.
I recommend mailing at least twice, but I think 3 times is a good number.
We've tried a couple of different combinations and found that 3 times works best. We did this on Easter and had 220 first time guests.
We did this on a random Sunday over the summer and had 80 first time guests!
Bottom line: Mail more often to a smaller list.