It depends on how much you do the jobs,maybe you do only 1-2 jobs a week. If you milk jobs everyday you get atleast 2 EC each day
Every 2 1/2 bars I complete an ec drops but u must not fail on any of the jobs for it to work so ur best bet to get them to drop is by doing job 1 over and over
honesly, if youre worried about ec that much, go out and buy some. 10 for a dollar isnt bad, but just dont go nuts