Sleeping through, I think, is considered as anything more than 6 hours.
My LO started "sleeping through" at about 6wks, in that she would sleep for 6 or 7 hours straight. We would put her to bed at midnight when we went to bed, and she would sleep until we got up. At about 10 weeks we started giving her a bed time of between 7pm and 9pm. She wouldlthen wake for a feed, because she would be sleeping longer, but gradually she would start to sleep straight through until 7am, and incredibly, last week she slept from 5pm to 8am (15 hours). She woke a few times and shuggied about her cot, but she wasn't crying for a feed/change, so I left her.
I personally consider sleeping through, sleeping until a sensible time in the morning (anytime from 6am). If she wakes, and manages to get herself back to sleep, I still consider it sleeping through, even if she's awake for an hour. If she needs a feed, but is quick to go back to sleep, then I consider it sleeping through - basically, anything where she doesn't fuss.
The night she went down at 5pm, she woke at about 7pm and again at 9pm, and I put her dummy back in and she went straight back to sleep. At 10pm she woke for a feed, which I gave and she went straight back to sleep. She woke a couple of times (during the early hours) but I didn't go to her, because she was only having a kick about in her cot, and she woke at 8am. I considered that sleeping through.