While in the Learner's stage I don't think there is a time limit because you always need to have someone with their full license of 3 years or more being the only front seat passenger. Keeping in mind they need to have a BAC of zero at all times to supervise.
While in intermediate it's different. You only need a supervising driver after midnight if you have more than one passenger
https://www.mpi.mb.ca/en/PDFs/GDLGuide.pdf