Friendships.... When Is It Too Much?

Probably everyone at some time has had a friend who required A Whole Lot Of Effort. I know the old adage "To Have A Friend You Must Be A Friend" is true, but sometimes a friend requires you to put in a whole lot more work than said friend seems to be putting in. Sometimes I just wonder where the line is when you have to put your foot down and just say NO. There's got to be a demarcation between being a good friend, and being a doormat doesn't there? I struggle with this.