You know it has been a long time since you have posted on your blog when:
Your url doesn't come up in your history
When you can't remember exactly what your url is
When you have to find your blog via a search engine
When you (finally) get to your blog and you forgot that you even posted the last post
When you are talking to no one when you blog because it has been so long people don't even come to your blog and check it unless directly told to...