As Val said, the length of stay will be determined by the Customs & Border Patrol agent at the port of entry. But I haven't heard of any cases where someone with a B1 or B2 visa wasn't granted a requested 6 months stay.
Like Val, in the years I visited the US from the UK, I was never asked to show a return ticket. But, since we've lived in the US and have been US citizens since the 80's, that is very old data. Undesirables aside, the main thing they're concerned about is that you'll be self-sufficient, and don't plan to stay here living off the state.
FWIW one time I stood in line at a US Consulate office in Canada, and an English guy in front of me was pleading to be granted a visa for entry to the US. He showed his return ticket, and was told "that doesn't prove that you plan to return to the UK". For such cases, some folks carry proof of a home, family and other commitments in the UK.
Over the years, anecdotally we've heard/read of a very small number of cases of folks not being granted entry/re-entry for some reason or other. But usually there's more to the story (e.g. a British couple making multiple successive trips to Canada, attempting to get around the system and get multiple 6-month extensions to their stay).
The comment you've read elsewhere, if true, merely confirms that the CPB officer at the port of entry has the authority to grant or refuse entry.
As always, I urge you to read the latest rules on the official US Government web pages linked in the sticky messages at the top of this message board.