Personally, once I"m as low as 20 big blinds, I"m actively looking for opportunities to get my stack in as a re-steal or a squeeze. I rarely voluntarily allow my stack to dwindle below this.
If down to around 8 to 12 bigs (dependant on antes, stacks behind me, etc) I"m willing to open shove most of the time in late position with any two cards.
Less than that, and I"m open shoving in any pot that"s folded to me.
The stack size I hate most of all is 20 to 30 big blinds - in most spots you"re playing for your stack here, but it is rarely correct to shove.