At Twitter, RSUs are subject to a 4-year vesting schedule: 25% vests in the 1st year (8.33% every 4 months), then 25% in each of the 2nd, 3rd and 4th years (6.25% every 3 months).

Every year employees get a new grant that vests over 4 years to bring them up to their target compensation. The number of vests in the first year may be variable. In some cases employees may have just 1 vesting date in the first year while others may have 3 vesting dates (6 month cliff, then quarterly after).