6 months for the 401k (aka Pilot Retirement Savings Plan)
essentially 12 months for the B plan (aka Pilot's Money Pension Purchase Plan)
excerpt from the Pilot Benefit Book on B plan eligibility
If you were a crew member on May 30, 1999, you automatically became a plan participant on June 1, 1999. Otherwise, you automatically become a plan participant on the first day of the month coincident with or next following:
The attainment of age 21; and
The first anniversary date of your employment with a Controlled Group Member, if you are credited with at least 1,000 hours of service during your first year of employment. See "Scope and Guidelines" for a listing of Controlled Group Members and “Hours of Service,” below. If you do not complete 1,000 hours of service during your first employment year, you may do so during any Plan Year starting with the first Plan Year beginning after your date of hire. You enter the Plan on the first day of the Plan Year next following the Plan Year during which you completed the required 1,000 hours of service.