Scrum has a method called Velocity. Its goal is to predict the speed of your team for the next Sprint.
First, you calculate how many man-hours you will have this week. Lets say, 30 hours. Second, you predict how much time each story is gonna take, and write it on the Card like that (in Trello):
I personally use [0] for 20 minutes, and then [1] to [5], respectively 1 to 5 hours (or more than five, but you should break user-stories that take more than 2 hours anyway).
Once you’ve done that for each Card in the product backlog, you take as any Cards as you can, given your available man-hours.
What’s interesting now is to compare during the sprint how much time you still have and how many Cards you have left. You can then predict if you’ll finish the sprint or not.
And at the end of the sprint, you will know how many man-hours you can really achieve, and be more accurate in the number of Cards for the next sprint.
Burndown Charts
For easy comparing, Scrum recommends Burndown Charts drawing this chart everyday helps you see the ideal sprint (blue) and your sprint (red).
Wikipedia’s links :
A good .pdf about burndown charts
“suppose I start work on time but then go for coffee while waiting for my computer to boot up”
Mountain Goat Software\’s explanation.