Don't make the beginner's mistake of running your ad in a single issue as a "test" and then judging your impact from that. Instead consider two factors:
- Opens are not the same people issue after issue. Not many recipients are inflexible in their reading habits. Some issues they'll open and some they'll never get to. A newsletter with a consistent 30% open rate probably is opened on a routine basis by at least 50% of the readership. If you advertise only once, you don't reach them all.
- Multiple impressions get higher response rates in every media ever studied, from TV ads to banners. The "perfect" number of impressions per prospect is roughly seven before response rates start dropping. You'll want to test to discover the sweetspot for yourself.
The best frequency for your ad is multiple ads clumped together as closely as possible for that multiple impression hit. If you can only afford to buy a few issues, don't space them out over a long period. Clump them.