Basically, depression can be "situational" for roughly a period of six months. After six months, it begins to look more serious and persistent. "Situational" depression might not have biological concomitant, but the longer you are depressed, the more your physical biology changes to adapt to the depression. However, this really requires professional judgement, if for no other reason, because there are many physical illnesses that produce depressive symptoms, such as thyroid problems. When I passed my licinsing exam, there was a movement to stress symptoms that suggest physical problems that mimic depression or other disorders, and encouraging psychologists to refer them to physicians for medical checkups.
Check out www.behavenet.com for the symptoms used to classify disorders, but as I said, it really requires professional judgment.