VoiceThread doesn’t restrict videos in terms of length but in terms of size. Any type of media file you upload to a VoiceThread presentation is limited to 3GB. If you have videos that are larger than 3GB, you could consider breaking them into smaller segments and uploading them to separate slides.
Remember that as a best practice, it’s good to limit video segments to 5-8 minutes in length. Also, having a series of shorter videos rather than one long one will make it much easier for students to return to a specific segment or topic that they want to re-watch.
NOTE: Mobile devices are great for recording video, but many of them record in high definition by default, which creates very large files. Try adjusting your device’s settings to record at a lower resolution to create smaller files that will be easier to add to VoiceThread.