My rule of thumb is this:
Will I do anything with it realistically? Do I have the money to put into it after I get it to make it usable again?
Once those questions are answered, I usually try to set a timeline/deadline for getting it finished. I hate to be a "project starter." If I do have something, and I keep it more than a reasonable length of time without doing anything to it, or without a plan, I then try to pass it on to someone who will.