It really depends on how big a credit you are giving and what is the length of plan the new customer must sign up for. I can't see many people referring customers and waiting months for a credit. Seems that 30-45 days would be acceptable.
What I do when a customer refers somebody to us is:
- For monthly payment plans, I give the initial credit when the new customer makes the second months payment.
- For annual payment plans, I give the initial payment after 30 days. This ensure they will not be taking advantage of the 30 day money-back offer after I give a credit.