No, there is no specific limit and you are welcome to play backing tracks of any length.
One thing to keep in mind is that long backing tracks may result in big file sizes, so please ensure that your hard drive does not become too full.
Track loading times can vary based on the length of the track. If you're streaming from Dropbox, please ensure you have a strong internet connection. If you're loading the track from a USB flash drive, be sure to allow plenty of time for it to be imported correctly.
Instructions on how to import a backing track from a USB flash drive can be found HERE.