As short as is practical. Use Alpha numeric to help identify modules from long names. There is no need to prefix modules in line with DISCO. It is better to align them with functional areas
2.01-01a Space saving for User Facing Modules: User facing modules can be renamed to a more descriptive name if required to save on dashboard space, but be mindful of not cramming too much onto a dashboard as per the general design principles
Related to Rule:
4.02-10 Turn off the Module Name
Best Practice article:
Questions on this topic? See 2.01-01 Naming Convention in our Planual forum.