Nothing wrong with a 12 hour shift if it's earning' your own wage.  

That said, you need a good work life balance.  Too often I watch these wife swop type programs and you see a very nice house and possessions and then under the surface it turns out one of them works 15 hours shifts to pay for it and never see's the kids/wife/husband.  Some "nice life" 
So I'd say, whatever you feel comfortable with and whatever gives the best balance between income and lifestyle.