Section 520.410 - How long does a messenger, learner, or apprentice certificate remain in effect?(a) Messenger and/or learner certificates may be issued for a period of not longer than one year.(b) Each special apprentice certificate shall specify the conditions and limitations under which it is granted, including the periods of time during which subminimum wage rates may be paid pursuant to a registered apprenticeship program.(c) No certificate may be issued retroactively.(d) The Administrator may amend the provisions of a certificate when necessary to correct omissions or defects in the original certificate or reflect changes in this part.