You have to have five years of actual federal service to be vested for federal retirement benefits. Other employment – even active duty military service for which you’ve made a deposit – doesn’t count, nor does unused sick leave.
However, those five years can be full-time or their part-time equivalent. For example, if you worked a part-time schedule of 20 hours per week, it would take 10 years to be vested.
If you leave before the needed five years of credit, you won’t have any title to an annuity. If you don’t intend to return to government service, you could take a refund of your retirement deductions, since that money otherwise would be lost to you. (Note: If you later changed your mind and returned to the government, you could recapture that time by paying it back, although the required interest charge could be substantial.)
Once you’re vested in the retirement system, you are eligible for an annuity. The only remaining question is when you’ll be able to claim it. If you’re looking for an immediate unreduced annuity, you can retire at age 62 with as few as five years of service, at age 60 with 20 or at your minimum retirement age with 30. MRAs range between 55 and 57, depending on your years of service. (Note: MRA applies only under the FERS system, not CSRS, but by definition new hires come in under FERS).
If you are willing to accept a reduced annuity, you could retire at your MRA with as few as 10 years of service. However, your annuity would be reduced by 5 percent for every year you were under age 62. You could reduce or eliminate that penalty by postponing the receipt of your annuity to a later date.
On the other hand, if you just wanted to put in your five years and then leave government, you could apply for a deferred annuity at age 62. That annuity would be based on your total years of service and your highest three years of average basic pay on the day you left. That means it will lose value to inflation over that time.
See also, Retirement Eligibility Under FERS and CSRS – MRA at ask.FEDweek.com