Upvote:2
The relevant threshold is 24 hours, as documented in point 2.5 and 2.6 of this document [PDF].
As you probably know, there are also other conditions, depending on your citizenship but travelling to the US with a US visa, it seems you fulfill them. Your friend might have needed a visa for one of these other reasons. See Is there a way to find out if I need a transit visa for a layover in the UK? for more info on all this.