I've watched a couple of games where people play against stockfish level 8 on lichess, and they pretty much lose all the time. Is it possible for someone to beat a level 8 computer in any time between 3+0 and 15+0?
Assuming Stockfish level 8 is full strength Stockfish, it's possible to beat it but extremely unlikely.
In very rare cases, Stockfish will incorrectly evaluate a complicated position, unless given a minute or two to think. If a human were to get in such a position in a 3 0 game, Stockfish could make a fatal blunder.
However, the chances of the human making a mistake first is far greater.
Making the time control longer than 3 0 favors Stockfish, since it has more time to think if it's misevaluating something.
If Stockfish level 8 is only 2200-2400 (I've read this on some online sources), then beating it is more realistic.
IMHO playing against Stockfish level eight is a lost cause when playing against its full 16 quad core processors and basically unlimited flash storage, no matter the time constraints. Magnus Carlson stated a couple of years ago that he always lost against that computer program. The lichess version is but a shadow of its real self.
Stockfish 8 CANNOT be beaten by a human in such fast time controls. Even strong GMs will face immense difficulty. It is definitely not impossible but it is highly improbable.
This book lists blitz/rapid games won by a human against Stockfish:
My winning games against Stockfish and Komodo. This is for real. I have played over 50 thousand engine games, of which more than 10 thousand against Stockfish, and a large number against Komodo, so I have my fair share of wins. Read the book to learn what openings are suitable to handle the machines, how to impose your style of play, and how to approach the game in a tactically relevant way. The games are amply commented and diagrammed. You thought competition between humans and machines has already ended? Not at all, it has just started!