What is the meaning of: "Friendship is a pretty full-time occupation if you really are friendly with somebody."
The post above does a good job of summing up this quotation. I'd add that this is a rather narrow definition of being "really friendly" and suggest that it's possible to maintain strong friendships without spendings hours every week with someone. Time must be spent and effort must be made, but I don't believe we have to all the way to "full time" with it.
I interpret this quote this way:
Friendship takes work, as any relationship does (family, significant other, etc.); therefore, enough time must be spent on it to help it grow and flourish and be genuine and healthy.