The team can decide to estimate in anyway they feel confident - Me
Although the team can decide it is good to know standard ways in which teams work. An industry standard is using Story Points.
Story points represent the relative sizing of a user story. It is designed as a rough estimate taking in a few key qualities to size the story.
The best way to estimate is to compare in size. Story point uses the Fibonacci sequence (1, 2 , 3, 5, 8 , 13, 20, 40, 100) to illustrate the relative effort of a story. A story point is made up of a combination of qualities:
Volume – How much is there?
Complexity – How difficult is it?
Knowledge – What is known?
Uncertainty – What is unknown?
Volume + Complexity + Knowledge + Uncertainty = Effort