When should I cut a mailing list from a direct mail campaign?

[George Rodriguez]

I've started a mailing campaign with two separate mailing lists. So far I've got the following results. The number of mailings include 1st and 2nd mailings. I'm planning to send out 3rd, 4th and 5th follow ups.

List A- 908 hand addressed envelopes= 9 calls ( .99% return) 2 closed deals (.22% return with a 30k + profit possibility) 

List B- 623 hand addressed envelopes= no call and no closed deals thus far. Only one call to take off mailing list.

I'm considering stopping the mailing campaign to List B but it might still be premature. And/or I can increase List A although there is a limited number of addresses to mail to when the intention is to close more 15 deals in the next 6 months.

What is the number of minimum mailings to heavily consider canceling a campaign?