In Agile, estimating the project duration requires the estimation of the size to happen in first place. Size estimation of a user story or a feature or the entire project can be done either by story points or ideal days. Mike Cohn dealt this in great length in his book “Agile Estimating and Planning”.
Team estimates the size based on their experience (compared to expert judgement in PMBOK), analogy (compared to analogous estimation) or splitting a big story into smaller pieces and estimating it (compared to decomposition in PMBOK).
Let us see very high level overview of these two agile estimation methods:
Story points – Story points will be assigned to each user story based on its nature expressed by the team (big/small, complex/simple, risky/straight-forward, etc). Story points are relative i.e. simple user story carries 5 points, complex one (twice that of simple one) carries 10 points.
Ideal days – Ideal days are ideal number of days taken for a user story based on the assumption that the team works on the user story and only that user story (without any interruption).
In general, Story points are widely used as a measure of size in Agile but it is left to the team which one they want to use – Story points or Ideal days – based on their need.