Some effects can arise surprisingly quickly. Even a single short meditation may, for example, influence your current perceived level of stress. But that does not mean ten minutes of meditation has produced lasting changes.1
Other effects develop only through regular practice. Studies therefore often examine programmes lasting several weeks. But they cannot predict when you personally will notice a change.2
The frequently mentioned eight weeks are not a magic threshold. They come mainly from the fact that many scientifically studied mindfulness programmes last about that long.