Duration of Shiv'a (and Shloshim)

Shiv'a lasts seven days. Shloshim lasts 30 days, beginning with Day 1 of shiv'a. There may be some exceptions if shiv'a occurs before or during festivals.   
Note Any part of the first day is considered to be one full day. On the final day after shacharit, the mourners finish shiv'a, so shiv'a can actually last as little as 5 ½ calendar days.