Ours is 6 months worth of expenses but I thin if your just starting your first goal is usually 3 months.
What are some common rules to live by?
Biggest one I think is pay yourself first. Really there are a ton of these though. If your employer does a 401k match make sure you contribute enough to take full advantage of it. Try not to carry any cc debt. Pay extra on your home and auto loans. I guess you could sum it up by saying "live within your means."