Educate Me - Netiquette 2

This situation brings up a good point! I get added as a friend all the time. No it's not because I'm popular or every one is reading my blog. How can I tell you ask? Pretty simple they leave a trail of spam behind. They add me and want me to visit their sites that have no meaning for me. Sorry, but they don't. These guys are collectors. There is no relationship building and this causes me to ask questions.

When is it ok to add someone's link to your page? Is it after one comment? Is it after two? What's the netiquette here?