"Rule of 3"
If an infant cries at least 3 hours a day, 3 days per week, for 3 weeks in a row, then the infant is considered to be suffering from colic.
Colic's Course
Colic usually starts when the baby is two weeks old and ends when the baby is one hundred days old (approximately three and a half months of age). Colic often peaks at six weeks and then begins to dissipate.
Parent Observation
The sound of the colic cry is intense - often described as a piercing, relentless scream. It can stop as abruptly as it starts and is often heard during or immediately after a feeding (when it is due to a gastrointestinal problem). The baby's face may be twisted and strained, causing onlookers to be highly concerned.
Witching Hours
Often worse at night, bouts of crying in the evening hours are often described as the witching hours.