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.