How long is too long to have PPD and does it fall under clinical depression?

Most physicians. Most doctors view post-partum as a clinical depression lasting from birth to 6 months post delivery. After 6 months and up to 1 year though is not uncommon for these symptoms to correct. After 1 year it is most likely a Major depression.
Ppd depression. Yes if it is beyond the usual duration of ppd. Treatment is the same but more important as it involves the life and wellbeing of the child. Environment issues need be analysed = if bad and toxic environment [family problem, husband not supporting and even brutal-abusive...] then it is not PPD but sheer depression as it is caused by mother's incapacity to tolerate environment.