I've worked frequently where 40 hours is long in the rear view mirror by the end of the week, but it's completely unsustainable over a long term period. I saw a few friends I'd not seen for a while and when they asked what I'd been up to all I could think was "um, work?", which completely defeats the point of working. Building something awesome is, well, awesome, but for the most part you also need to have a life and enjoy yourself.