How much buffer should we allow for in a sprint ?!

Was in a coaching session today and a discussion about buffers within a sprint came up.

Why do we need buffers?

Why do all buffers always get consumed much before the project deadline ?

Do we buffer tasks or do we buffer a project ?

Does the scrum way of task management and release management align well with buffering best practices?

Book Suggestion : Critical Chain By Eliyahu Goldratt