This is a configurable Placement setting. Edit Placements from the Platforms section of the Developer Dashboard.
1. On the Developer Dashboard, select your Project.
2. Select the Operate tab, then Platforms from the left navigation bar to view a list of your Project's active platforms.
3. Select the platform you wish to edit Placements for.
4. Select the AD PLACEMENTS tab to bring up the Placements list, which displays the ALLOW SKIP setting.
5. Find the Placement you wish to edit (or create a new one), and select EDIT to bring up the configuration window.
6. Set the Allow skip after field to specify the number of seconds you want the ad to play for before allowing the user to skip it.
NOTE: Skipping is disabled by default for the rewardedVideo Placement.