Dan's Answer: Generally:
Deferral must be at least 5 years from the date of the original vesting.
Deferral must be made at least 6 months prior to the expected original vesting date (if vesting is performance based)
Deferral must be made be made within 30 days of award, must not vest for at least 12 months from the election, if vesting is time/service based.
Redeferral ( after the first 5 year period) generally not allowed. If allowed, it is normally for another 5 year pear period.
Some companies include the ability to accelerate payout in the event of death. This will void the deferral of income if triggered.
From Quora: http://www.quora.com/Restricted-Stock-Units/If-an-RSU-plan-allows-for-deferral-of-receipt-of-shares-at-the-option-of-the-recipient-what-are-the-frequent-parameters-terms-and-restrictions-of-that-deferral