The best approach is to begin right away and distribute a few hundred (At least) per week to get a good sample size.
Then, keep track of the number of people who call and average it with the number of flyers you're getting out. When that number starts going up, it's time to get out as many flyers as you can and keep it up until the call volume drops off.
You have to listen to the market. It will tell you when it's ready. This is the best way to go about catching the market while it's hot yet not wasting too much time or money before the market is ready.