Is there any rule on the maximum task duration on a project schedule?

No, there's not, it depends on the Project and it depends on the Project Manager. However, the granularity level of the work packages should not be too much detailed nor too much general, it should be something in between.

Too detailed: Any blimp in the project schedule and you'll find your whole gantt chart in need of a redo.

Too general: You gantt chart will be of no use.

Hope my answer helps.