Would working 7 days a week for 6 months straight be tiring for you?

I hear ya. I am one of those overworked truck drivers who transport the auto parts. Tonight, I begin another 14 hour shift. 

Yes.  Anybody would burn out under those circumstances.  That's an abusive labor practice (which are common in manufacturing) and the reason they're replacing people with robots.  Let them abuse robots instead.

Where I live there is a working time directive written into law so working 56 hours a week would be illegal

Clearly not too much for the employer to expect, if you're putting up with it.

Not trying to seem like a smartazz here, but an employer has every right to try to lower his manpower costs any way he (or she) can, including keeping the line open 24/7 and running it with the same three shifts instead of hiring a fourth or fifth shift.  The employer's choice is to do that while paying American workers a wage that's sufficient to maintain their standard of living OR moving the entire manufacturing line to another country where the workers have a lower standard of living and will work for far less money.  Over there, the employer could hire additional shifts and STILL produce the parts at a competitive price.

Unfortunately, your choice comes down to over-working for income that will support the lifestyle you're accustomed to, or quitting...which will probably result in you taking a job with better hours, but that will NOT support your lifestyle - and you'll have to make sacrifices.  So...do you want the cash for your motorcycle or skidoo or the annual cruise with wife & kids?  Or do you want time to yourself, without the extra cash, (or without the retirement account, or whatever you spend it on)?  Due to pressure from Chinese and other competitors, this is becoming the reality for all American factory workers.  There's no avoiding it.  It's just your call to make.

I suppose that depends on how much vacation time you get and how well you are paid. Typically, that is an unreasonable and grueling schedule when physical labor is involved.