Song Length vs Quality

A teacher was mentioning this last semester,he told us: well now you don't know what a song more than 4 minutes is like,Iron Butterfly was our anthem even in the radio version with 7 minutes.

I like some long songs but sometimes a very long song bores me to tears because I just like a certain part
 
I personally love long songs, but only if they are successful. Opeth has become masters at this art, but on the first two albums they suffered from the stop and start a new riff syndrome. Black Rose Immortal is the epitamy of this problem. I know many believe this to be the greatest Opeth song ever...but I tend to think that only because it is the longest, which is really what hinders it. It is the only Opeth song that I really think was just absolutely poorly done. It has some decent moments, but nothing incredible. Sorry to rant, but I hate a long song that is poorly done.
A long song, should not feel long.
Enslaved - 793
Moonsorrow - Sankaritarina
Apotheosis - The Maimed God
Graveland - Sons of Fire
Bergthron - Ulrate Gedanken
These are perfect examples of long songs, that take you to another world and back in 15-20 minutes, but are capable of making it pass with ease.