There is a wonderful article about one day tasks that I strongly recommend to read:
One day tasks are tasks that take -in theory- up to one day to complete by one person, or by a pair of people in the case of pair programming. Creating tasks of one day maximum size gives two benefits to the team. The first is that under normal conditions it creates visible daily flow. Meaning that if nothing goes wrong, one task will be finished (on average) per team member per day. So if a team has 8 members, theoretically they should finish 8 tasks per day. But in Agile teams, people work in pairs a lot, either pair programming, or a coder/analyst-tester pair, so in practice it should be around half of that that gets finished. And effectively, my experience is that in healthy 8-member teams, you should expect to see around 4 DONE tags per day.
Read the full article here: One day tasks.
But … I do not agree anymore to this idea. What I discovered in several teams during the last 2 years is that it is much simpler to help people to understand that they need to decompose tasks while they are working on. So a task that gets not done after started within one day, must be broken down into smaller units. And then you start to work on a remaining task of the big old one. Maybe, you will not finish this task, so decompose the task again on the next day until it is completly done.