It depends on the job description in individual jobs - if, for example, you spent those 20 years in the same or similar job (in one or closely related professions), it probably doesn't matter to the future employer. But if you have changed not only employers but also professions, try to be as accurate as possible. In practice, there is no danger of in-depth verification - at most, the (future) employer will request an opinion from the previous workplace (or two if the previous one was too short-lived). If for any reason you need to specify the duration of previous jobs, you can ask the CSSA to issue an informative personal letter (employers are obliged to provide the CSSA with information about employees and the duration of their employment) - once a calendar year.