How late is your period allowed to be?
It is normal at an early age to miss a month even several months, have 2 in one month. Some females never have what we would term as a regular period. This is why seeing your doctor on a regular basis is important to rule out any problems.
Depending on the girl...a period can last 1-7days. The average is somewhere in between 4,5 days. Also the cycle is different for some as well...some have their period 21 day cycle, 28 day cycle etc. If you have a period that last longer than 7 days see a doctor.
sometimes you may just skip a month