If you are still within the first year of starting your period it is normal for it to be irregular. After that most women's period is fairly regular but it isn't too uncommon to have some fluctuating, especially if your cycle is syncing up to someone else's or can be caused by things like stress.
It depends. I personally was known to miss up to 5 periods due to stress and not eating enough (personal problems). Some people have irregular periods also, especially if you're just starting or younger. Your period typically takes 2-5 years to become regular. If you're not sexually active, don't worry too much unless you have other symptoms. If you are, take a test and stay protected. If you're still worried, visit a doctor.