When is it time for a "panic all-in"?

Everyone's been there one time or another - in tournament and although you haven't won a pot in the last hour, the blinds keep creeping up on your. Everyone knows that at some point you need to put all your chips in the middle and hope for a double up. But when exactly is that time?

My rule-of-thumb is that you never want to spend too much time as a considerable short-stack. This doesn't mean that just because you have the least amount of chips you need to panic, but I use short-stack in terms of the Blinds - that in a few rounds you'll be eaten alive. What this means is, make sure you still have enough chips left that a double up will make you a legetimate condender. I used to wait too long, just praying for those pocket kings/aces that never came, meanwhile getting eaten alive by the blinds and forced all-in with J4 offsuit. What are you're thoughts on this?

Very recently I finished a 3-table sit-n-go just out of the money in 10th. I was down to 1200 in chips with blinds at 150. I was in the big blind, and with only the button calling and the SB completing, I decided to push all in with QJ suited. It looked like they'd both limped into the blind, and suprisingly enough I got a call from one of the larger stacks with Q7 offsuit. Needless to say he caught a seven. I would have been satisfied stealing the blinds...

Even though I lost the hand I thought this reinforced my belief of going all-in when you're still a formidable force in the tournament. If I'd had say 500 chips in that situation, I probably would've gotten called from both the big-stacks, and wouldn't have been such a favorite going into the flop.

Someone add their two cents about any of this.