Its a numbers game, for sure, and reality is sometimes it take a significant amount of time for people to call in. We've received calls from potential sellers many years after our mail is dropped, so you never know if someone isn't simply filing that away until they need it.
For me I have market data from HomeVestors that shows me cost per lead in a given area and also average conversion rates so easy for me to figure out how many buys I'm looking for in a given year relative to a specific market, and I'm active in 2. For you I'd say continue to advertise as you can until you feel comfortable with an average.
The key to direct mail is consistency and longevity!
Real Estate is a tricky beast sometime because of the dry spells we all go through. If you understand the averages in an area and are spending appropriately to be successful then when looking back at your year the numbers will align.
Always happy to chat about benefits of HomeVestors if you want since we have significant experience with this problem. Message me if you wish.
Happy Investing!!