The Buddha said “I teach one thing and one thing only. Suffering and the end of suffering.”
The Buddha called “the end of suffering” nibbana.
It’s really that simple.
What does it feel like?
The best explanation that I currently have is: the total subjugation of the “task negative network” by the “task positive network”.
Here’s a video which explains what those 2 networks are.
How long does it take? How long does it last?
My own experience is one of slowly developing a reliable sequence of incantations to skillfully reduce and then eventually snuff out suffering completely.
At first, the time to accomplish the goal was considerable and the state of nibbana did not endure for long.
However, with the passage of time, I’ve noticed both
1: the time to get to nibbana becoming shorter and shorter
2: maintaining the state for longer and longer periods of time.
The boring truth is that attaining nibbana is simply a skill like any other. The more diligently you practice, the greater your proficiency in the skill.
The suttas say that a state of “non-returning” is possible.
I have not yet attained that, so I don’t know if it is true.
My experience thus far suggests that it might be.