Doing a PhD with significant external time obligations (e.g., marriage) Graduate school has a reputation for being a tremendous time-sink. I'm currently married with kids, and I have a fairly busy extracurricular schedule. Is it common for married people - or anyone, actually, with significant external time constraints - to not finish their graduate work, specifically due to conflicting obligations?
The people who I've seen with considerable external commitments who have still gotten through in a reasonable amount of time all boil down to one thing: They treat grad-school like a job. From 9 to 5 (or 10 to 6, 11 to 7, whatever works for their schedule) they're working. No, they're not available for the infinitely many distractions of life, any more than someone at an office is. They pack a tremendous amount of productivity into that time, and then when they need to be with their family, they're with their family.

Those at least are the people who got through it sane. I think the other major trait is to recognize that, because you have a life, this might take a little longer, and not burn yourself out trying to hammer things through to the point of exhaustion.