How long is too long for a monthly cycle to last?

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I mean how long would you expect a period to last, and when would you start thinking maybe you should go to the doctor? Should your period only last three or four days, or is it okay for it to last nearly two weeks. on average. I am just wondering because the last two times I have had mine it has lasted a long time, but before that I did not even have one for three or four months.