Never really thought about it...but I would say 30BB is about right. Here are some thoughts of mine.
Less than 4 BBs: I am pushing allin with any hand that I would normally play from late position, basically any two face cards, any ace and any pair.
4-10BBs: I am looking for a premium hand to push allin (AA-JJ, AK, AQ). I will also go allin with any pair from late position if there has not been a raise ahead of me.
10-19BBs: I have a little room to play, but I play very solid and try to only
bluff if I am 80% sure I will take down the pot. But I have to admit that the c-bet has hurt me a few times with a stack this size. Very aggressive with good hands and no tricky plays. Will push allin if I feel I need to protect my hand against a calling station or concerned about a tricky player.
20-29 BBs: About the same as 10-19BBs, but willing to take a few more risks. Less likely to push allin.
30BBs or more: About the stack I need to feel comfortable with playing any aspect of my game.
Of course, these are all generalizations and probably a little more of how I THINK I should play then what I actually do in all situations. I still tend to be too aggressive without a great hand when short stacked.
Good topic.