Criteria for work life balance

Lots of people talk about wanting work life balance, but what does that mean? Most people aren't categorically opposed to doing work on weekends; they'd just prefer that it not be normal. So, how much is too much? Are you willing to miss a family event because something came up at work? For me, if it's my brother's wedding, the answer is clearly "no". But if it's missing a school play or a rare dinner reservation, it's debatable.

Recently, I've been trying to articulate a set of rules to help me decide when I'd say "no" to work. While answers on the extremes seem clear, the cases on the margin are hard to categorize. Given that those are more frequent, I'm concerned that without a heuristic I'm likely to err on the side of prioritizing work over my personal life.