Note: This functionality is only available in Boxer for iOS currently. To vote for this feature to be added to Boxer for Android, go here.
To change the amount of time that the 'Undo' option pops-up:
1. Open up the app settings.
2. Select "More" from underneath the "Mail" section.
3. Select "Undo duration."
4. From here, you can choose from four options for the duration of the undo option.
5. Once you make your selection, tap on the 'return' arrow in the top left corner to save.