The average period lasts for about 5 days. Some shorter, some longer. Many may have a few days of light spotting before or after this as well. So, what is too long? If you are bleeding heavily for a prolonged period, for more than nine days on one period, or your period consistently lasts more than a week, you should contact your healthcare provider to find out why. While some may just simply have longer periods, there are options such as birth control pills to help control this. The most common side effect of prolonged bleeding is iron deficient anemia. Any menstruating female can develop this, more so if you have longer lasting periods. Your healthcare provider can help you figure out if anything can be, or should be, done to shorten bleeding time (depending on the cause).
When it comes to our periods, we're all different. I, for one, have times when I'm really irregular and my periods can be heavy and long. For many years I just lived with the inconvenience. Finally, I spoke to my doctor and found out the reason for my irregular periods. If your period is lasting more than 7 days, then it's time to bring it up with your healthcare provider. Don't wait like I did! Like me, you may have an underlying condition that's causing long periods or there may be no issue at all -- only your healthcare provider can determine that for sure.
I'm not totally sure what you mean by "too long." Period length can vary a ton! I've had three- or four-day periods, but I've also definitely had seven- to eight-day periods. If you feel like your period is longer than usual, or you have any other symptoms, it won't hurt to make an appointment with your healthcare professional.