may be he works eight days, then has several days to rest?

I think it mean Eight-Hour per Day

it is hard to identify the meaning without the context. it could mean someone works for the next 8 days OR they might be referring to the expression "eight days a week" which just means that they are extremely busy

It could be any of those above, but they also be saying because they work too much. It like saying i need 8 days in a week to finish all the work i have. It just all depends in the context it is in.

He works eight days, then has a rest of a few days. It could also be an irony. He might mean he is so busy that he works "eight days a week". It's a Beatles song, by the way.