I wouldn't necessarily wait until I was left with 7 big blinds. It depends a lot on the position, the size of your opponents' stacks of chips, and the value of your fold equity.
With 10-15 large blinds (BB), you should already be considering push/fold options or looking to simplify your decisions; around 7 BB, many situations clearly turn into push or fold scenarios. The danger lies in waiting too long and going down to 3-4 BB, as your ability to force the fold of others plummets at that point.
For me, the key question isn't “How many big blinds should I go all-in with?” , but rather “How many big blinds can I lose before my stack loses its fold equity?” That's when you start to push more aggressively.