The length of the monthly cycle varies. Usually, during the first 5 to 7 years after your first menstrual period, the cycles are more irregular and the interval between them is longer than for cycles later in life. Typically, during your twenties and thirties, the cycles become increasingly shorter and more regular. When a woman enters her forties, the cycles begin to lengthen again.
So, how do you know if you're cycle is normal? An average cycle length is 28 days, but if your cycle lasts anywhere between ~24 to 35 days, it's considered normal. A normal period flow lasts anywhere from 4 to 6 days, causes an average blood loss of 25-60 ml, and can be light, moderate, or heavy. Flow lasting longer than one week and blood loss of more than 80 ml are considered abnormal. Also, passing the occasional blood clot is normal.
By convention, the 28-day cycle is considered the ideal cycle. (Only 10-15% of cycles last exactly 28 days.) This doesn't mean that if your cycle isn't exactly 28 days there's something wrong with you; 28 days is just an average. [Not to mention that it makes the math simple--in a 28-day cycle, ovulation happens at the halfway mark, on the 14th day.] Also, by convention, the first day of bleeding is considered the start of the cycle, and is denoted as Day 1; the period lasts for 5 days (in an ideal cycle), or from Day 1 to Day 5; and the cycle ends on Day 28. Of course, real life doesn't always conform to conventions: if you're cycle isn't 28 days, how do you number the days of your cycle? Let's use an example to illustrate.
Say your period starts on the tenth of the month, your bleeding lasts for 4 days, and your cycle length is 30 days.
The day the bleeding starts, the tenth of the month, is Day 1 of your cycle. The blood flow days are Day 1 through Day 4, or the tenth through the thirteenth of the month. Your cycle ends on Day 30, or the ninth of next month.
One more useful calculation: your ovulation day. (Ovulation = the release of the egg from the ovary.) In an ideal, 28-day cycle, ovulation happens on day 14. Obviously, if you're cycle isn't 28 days, that doesn't help you. Fortunately, ovulation day is remarkably constant from woman to woman, and you can calculate your probable ovulation date based on the length of your monthly cycle. Here's one helpful way to think of the monthly cycle: a cycle divided into two intervals. The first interval (preovulatory) lasts from the end of your period to ovulation. The second interval (postovulatory) lasts from ovulation to the start of your next period. The preovulatory interval can vary widely; it may last for four days, or nine days, depending on the length of your cycle. In contrast, the postovulatory interval tends to be fairly constant at 14 days, regardless of how many days your cycle lasts. Crystal clear, right? You can now calculate your ovulation day, even in your sleep if you have to. [Huh? - ed. Now you know how I feel when someone tries to explain tech-related stuff to me.] Let's go through this step-by-step:
First, you need to track your cycle. Mark the day your period starts; this is Day 1 of your cycle. Then count the number of days until your next period; this is your cycle length. Do this for about three cycles in a row (more than three is fine, less, not so much). [The reason you need a minimum of three months is because cycle length might not always be the same each month. After three consecutive months you should be able to see a cycle length pattern.] Once your fourth period starts, count forward from Day 1 the number of days in your cycle length and mark that date. Then, from that day, count backwards fourteen days. This is your presumptive ovulation day. [You never count forwards, from the start of your period, because the preovulatory interval varies in length. You count backwards, because the postovulatory interval is fairly consistent at 14 days.]
For example, let's say your cycle lasts 24 days and your best friend's last 31 days. Both of you will most likely ovulate 14 days before the start of the next period. For you, this means ovulation is on Day 10 of your cycle (24 - 14 = 10), while for your friend, ovulation happens on Day 17 of her cycle (31 - 14 = 17).
Keeping track of your monthly cycle isn't useful just for planning a pregnancy. It's also beneficial if you plan to manage your period. In particular, it's useful if you plan to use period control occasionally, like for a scheduled event. If you know you have an upcoming event (vacation, exams, business trip) and you don't want to have a menstrual period around that date, the best time to suppress your real or fake period is about three months in advance. The advantage: it lowers the likelihood of nuisance side effects, like breakthrough bleeding/spotting.