You start counting from the first day of your period.
It can take awhile for periods to get regular, though depending on when you started it should be about that time. Some people just have irregular cycles, but have you had a checkup and talked to your doctor or gynecologist about this? It would be good to rule out any health conditions, especially if when you do get your period it's unusually heavy or painful or whatever.
Yes, birth control could help regulate it, but if you don't want to take birth control that's a reasonable and valid choice, too.
It's possible that stress may make it late. It's also possible that you could just be one of the lucky people who has very irregular cycles. Either way, if it's bothering you (and it sounds like it is), it's time to talk to a health care professional about it.
"And when the answer that you want/Is in the question that you state/Come what may..." - Coheed and Cambria, "Blood Red Summer"