Table:
|  | Number of RSUs Outstanding (in thousands) | Weighted- Average Grant Date Fair Value Per Share | Aggregate Intrinsic Value (in thousands) |
| --- | --- | --- | --- |
| Outstanding at December 31, 2016 | 3,554 | $18.01 | $73,261 |
| Granted | 3,005 | 30.20 |  |
| Released | (1,680) | 19.54 |  |
| Canceled/Forfeited | (598) | 20.91 |  |
| Outstanding at December 31, 2017 | 4,281 | $25.51 | $207,197 |
| Granted | 1,746 | 67.64 |  |
| Released | (1,971) | 30.50 |  |
| Canceled/Forfeited | (495) | 34.99 |  |
| Outstanding at December 31, 2018 | 3,561 | $42.09 | $293,523 |
| Granted | 2,069 | 122.35 |  |
| Released | (1,906) | 50.99 |  |
| Canceled/Forfeited | (475) | 60.38 |  |
| Outstanding at December 31, 2019 | 3,249 | $85.39 | $548,145 |
Paragraphs:
Restricted Stock Units
The 2013 Plan provides for the issuance of RSUs to employees, directors, and consultants. RSUs issued under the 2013 Plan generally vest over four years. A summary of activity of RSUs under the 2013 Plan at December 31, 2019 and changes during the periods then ended is presented in the following table:
As of December 31, 2019 and 2018, there was a total of $198.3 million and $107.9 million of unrecognized share-based compensation expense, net of estimated forfeitures, related to RSUs, which will be recognized on a straight-line basis over the remaining weighted-average vesting periods of approximately 2.3 years and 2.4 years, respectively.
Question:
How many years do the RSUs issued under the 2013 Plan generally vest over?
Answer: four years