He's not sure when home stopped being a fancy manor on Osiris and became a cramped bunk on an old not-at-all-trashy firefly class spaceship. He isn't sure when he stopped waiting for the day he could go back to his old life, never having to want for anything.
He's not sure when eating flavorless protein mush for nearly every meal became worth it just to see her smile, hear her laugh. He's definitely not sure when spending nearly every bit of money he had on a ring that he'd never have looked at on Osiris became not only acceptable, but preferable. She wouldn't want the rings from the Core, anyway. And they might get stuck in Serenity's engine.
He isn't sure when calling Jayne the ape-man-thing became a joke of sorts, when Jayne stopped being an almost-enemy and became an almost-friend. He's not sure when following the captain stopped becoming 'ridiculous' and 'insane' and became second nature instead. He's not sure when it got to the point that there wasn't a thing in the 'verse that could make him want to return to the life he had before rescuing River.
He's not sure when he started seeing Unification as nothing more than an attempt to control people, to tell them what to do, how to do it, when to do it, and what to think. He's not sure when he started thinking that the Alliance as a whole is a joke run by fools who have no idea what they're causing.
He couldn't say when River stopped being someone he constantly had to protect and became someone who could easily kick his ass across the 'verse. And he's not sure when he became okay with that, and the fact that she's grown up. He's not sure when he let go of the past and started clinging to the present.
He isn't sure when he stopped wanting to bring back pre-Academy River and accepting the new River as she is.
He's not sure when sitting around a table with a bunch of 'petty thieves', eating flavorless protein mush, holding Kaylee's hand, hearing Mal tell Jayne to stop telling dirty jokes, watching River steal Jayne's food right off his plate, noting the not-so-discreet looks between Mal and Inara, seeing Zoë try to hide her amusement, he's not sure when that became something damn near close to heaven.
He's not sure when Serenity became home, but he wouldn't change it for the 'verse.