I mailed 10k pcs 2 weeks ago. Had a pretty steady call volume the first week. Still getting a few calls from those flyers as of today. I have 15k more but I'm holding those off until the weather breaks atleast partially. I have had a very minor increase in traffic to my website after the flyer distribution as well so I'm thinking people just aren't interested in their lawn just yet.
I don't expect to be applying until the 2nd week of April at the earliest. You're much larger than us though if you're getting 200 new customers per year. I have around 200 fert account total right now.