The essential calculation is how much a customer is worth over the lifetime of your relationship with that customer. Big marketers use lifetime value to plan budgets. To calculate projected value, using historical performance data, or the likely results from future marketing. Expect to show a profit in two to three years. However, if you are not a big fat company, or if you don't have enough historical data to calculate what a lifetime value is, you should aim to try and break even on your first sale - you don't make any money, but you don't lose any money either.
Advertising is the greatest art form of the twentieth century.