Can you tell me how quick can a person fall into depression?

A formal diagnosis: Of major depression - requires at least two weeks duration of symptoms.

Falling into a hole?: Everyone can get depressed. To meet criteria for clinical depression it has to persist for @ two weeks, and interfere with your functioning in some way. Here is a resource: http://psychcentral.Com/disorders/major-depressive-episode-symptoms/ there are different other diagnoses that may be occurring, so you may wish to talk with someone to explore these. People describe falling into a hole.