EPSO usually require the following professional experience: for general Administrators, on AD5 level exams no work experience is required; on AD7 level, at least 6 years of experience is required (for lawyer-linguists this is usually only 3 years); for AD9 level, at least 12 years is required.
For Specialists, it depends on the specific profile, but usually the same number of years is expected.
For Assistants on AST3 level, the rule is either having a secondary education diploma in the given field and 3 years' of experience, or any qualification giving access to post-secondary education and then 6 years' of relevant experience.
It is important to check the specific requirements in each Notice of Competition as it may vary from one exam to another.
See also:
Assistants | Administrators (AD) | Notice of Competition | Diploma Requirements
Related link:
EPSO | EPSO Competitions - A Look at the Different Selection Processes