how many cofees is too many?

When establishing sources and strengthening relationships with those that we need information from on a weekly basis, a cup of cofee with them was suggested to strengthen those ties, however it how many coffees with that person is too many. I think that the Journalist should draw a line between them and a source so as to ensure a certain degree of objectivity, how many cofees indeed does it take until the source becomes a 'friend.'

I think that once the journalist and the source sit down for more than two cups of coffee, undesirable expectations will come into place, thus interfering with the purpose of the alliance! Concepts such as integrity, burnt source and others come to mind.