This often depends on the industry. On the whole, technology and knowledge based projects should probably have a length of no more than 1-3 weeks. Construction industry tasks can usually be scheduled for longer periods of 3-6 weeks. Another rule of thumb suggests somewhere between 1 and 10% of the project time.
So how do we arrive at these recommendations? One needs to examine factors such as:
- how well defined is the work (better defined means the tasks can be longer)
- how certain is the team of their estimates (higher certainty means tasks can be longer)
- how often you are able to check status (if during a status meeting an issue is found, it can take up to two more meetings to determine and confirm the solution)
- how critical are the tasks to project completion (tasks on the critical path should be shorter to maintain better control)
To avoid an unmaintainable project schedule, be sure to use the dependencies to create flexible project plans such that a change to one task automatically updates the others.