Usually it's within a day or two. Sometimes it's different depending on the woman and her system, stress level, etc. But in general, with a couple of days after the 21st pill (last before the week of no pills).
Most women get their period on the second or third placebo pill (or pill-free day). However, getting it ANY time during the fourth week is normal. Some women do not even get their periods until the six or seventh day of the fourth week, and it is perfectly normal.