My dd is 13 months and has never been a great sleeper. Her best was about 5/6 hours without waking for a couple of nights in a row, but more reliably it would be 2/4 hours between wake ups. She used to spend the whole night in the cot, but since not being very well for a month or so (worse cold than usual that became a chest infection coupled with teething) she is fine for the first 2-4 hours but then we simply cannot put her back down and she won't settle unless I take her into bed with me. She isn't properly awake at this point - it's not that she will only go back to sleep next to me. She's gone to sleep but doesn't seem to get into a deep enough sleep to be able to put her down.
Everyone I know has told me to put her down 'drowsy but awake' but this has always just meant she immediately wakes up!