"a decade" means 10 years. So "decades" mean many periods of 10 years; it is not specified how many. "I've done for" indicated Present Perfect tense which is used when something began in the past but continues until now. For example, I started studying psychology in 2011 and I am still doing it so I would say "I HAVE STUDIED psychology FOR 9 years. Summing up, "I've done (it) for decades" means that I started doing something many years ago (more than 10) and I am still doing it. "Left" in this context refers to free time that he had. For example, I should be at a meeting from 1 to 2 pm but the meeting ended at 1.45pm, so I can say "I spend 15 minutes that were LEFT eating my lunch". If you have more questions or you want to work on your English, please feel free to contact me.

'do' in this sense is habitual. Do you wake early? I do, I have done so for as long as I can remember. It is time to have done with this topic!

oops! Sorry, I posted my answer twice so I removed the one here. See the answer below.